The Pennsylvania State University. The Graduate School. College of Earth and Mineral Sciences

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1 The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences EVALUATING CONTROLS ON CREVASSE-SPLAY GROWTH IN MODERN AND ANCIENT FLUVIAL SYSTEMS A Thesis in Geosciences by Craig L. Millard 2013 Craig L. Millard Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2013

2 The thesis of Craig L. Millard was reviewed and approved* by the following: Elizabeth A. Hajek Assistant Professor of Geosciences Thesis Adviser Eric Kirby Associate Professor of Geosciences Rudy L. Slingerland Professor of Geology Chris J. Marone Professor of Geosciences Associate Head of Graduate Program of the Department of Geosciences *Signatures are on file in the Graduate School ii

3 Abstract Crevasse-splays and related facies are common in some avulsive fluvial systems, while being nearly absent in others. As such, the study of splay and other overbank deposits may be fundamental to understanding the processes of fluvial avulsion and basin-filling in terrestrial systems. Despite this importance, the primary controls on crevasse-splay growth and development are poorly understood and constrained. Within this study, the role of floodplain drainage and channel grain-size distribution on crevasse-splay growth is examined, as these variables influence the amount of sediment and water discharged through levee crevasses. To evaluate the influence of floodplain drainage and channel grain-size distributions on crevasse-splay growth, three approaches were used. First, crevasse-splay extent, plan form, and frequency were documented using aerial photography from Google Earth in four modern, avulsive river systems including the Ovens (Victoria, Australia), Sandover (Northern Territory, Australia), Saskatchewan (Saskatchewan, Canada), and upper (British, Canada). Second, field observations and measurements from the Paleocene Fort Union Formation (Bighorn Basin, Wyoming), Paleocene-Eocene Willwood Formation (Bighorn Basin, Wyoming), and Late Cretaceous-Paleocene Ferris Formation (Hanna Basin, Wyoming) documented floodplain drainage and grain-size characteristics in these ancient systems. Finally, numerical modeling using Delft3D-FLOW was conducted to explore splay growth under different floodplain drainage and channel grain-size conditions. Collectively, modern observations, ancient deposits, and modeling results show that systems producing wide-spread, basin-filling crevasse-splay deposits are associated with welldrained floodplains with steep cross-floodplain water-surface gradients and abundant intermediate grain-sizes (i.e. coarse silt to fine sand, which is readily suspended in channel flows and also settles from suspension quickly as through-crevasse flows expand and decelerate on the floodplain). In contrast, splay production was limited in systems with poorly-drained floodplains and/or coarse grain sizes. Some of the systems featured little or no crevasse-splay deposition (e.g. Ovens River and Ferris Formation) or numerous splays with relatively little depositional volume (e.g. upper and Sandover Rivers). In these systems, splay deposition is limited because standing water on the floodplain reduced cross-floodplain water-surface slopes, iii

4 suppressing sediment advection away from the channel margin, and/or these systems lacked intermediate sediment sizes that supply crevasse-splay sediment to the floodplain. Ultimately, these results suggest that well-drained floodplains with steep, lateral watersurface gradients promote extensive crevasse-splay growth. The influence of channel grain-size distribution is less pronounced than floodplain-drainage conditions, although abundant "intermediate" sediment supply (i.e. coarse suspended sediment) promotes large crevasse-splay deposits. iv

5 Table of Contents List of Figures... vii List of Tables... x Acknowledgements... xi 1.0 Introduction Background Controls on Splay Development Primary Controls on Splay Area and Volume Relationship of Splays and Fluvial Avulsion Splays within Progradational Avulsion Systems Splays within Incisional Avulsion Systems Crevasse-splays in the Stratigraphic Record Hypotheses and Study Phases Hypotheses Hypothesis I Hypothesis II Study Components Observations in Modern Rivers Study Areas Upper River Saskatchewan River Sandover River Ovens River Google Earth Observations and Measurements Google Earth Study Results General Appearance of Splays Analysis of Splay Shape Splay Frequency and Basin Filling Splay Sizes and Shapes Splay Volume Estimates Discussion Observations in Ancient Systems v

6 5.1 Formations Ferris Formation Fort Union Formation Willwood Formation Field Observations Results Ferris Formation Fort Union Formation Willwood Formation Discussion Modeling Splay Development in Delft3D Methods Model Grid and Initial Topography Initial Bed Sediments Initial and Boundary Conditions, Including Water and Sediment Discharge Results Run 1: Fine Channel Sediment with Dry Floodplain Run 2: Fine Channel Sediment with Wet Floodplain Run 3: Intermediate Channel Sediment with Dry Floodplain Run 4: Intermediate Channel Sediment with Wet Floodplain Run 5: Coarse Channel Sediment with Dry Floodplain Run 6: Coarse Channel Sediment with Wet Floodplain Discussion Synthesis and Discussion Conclusions References Appendix A: Splay Polygon Raw Values Appendix B: Flood-basin, Channel, and Island Measurements Appendix C: Values from Representative Ancient Grain-Size Distributions Appendix D: Summary Statistics for Studied Outcrops Appendix E: Location of Data Files vi

7 List of Figures Figure 1.1: Splay Development Schematic Figure 1.2: Splay Length vs. Channel Width Figure 1.3: Splay Width vs. Channel Width Figure 1.4: Splay-Deposit Thickness vs. Channel Width Figure 1.5: Comparison of Saskatchewan and Amazon Splays Figure 2.1: Floodplain Drainage Types (after Adams et al., 2004) Figure 2.2: Progradational and Incisional Avulsion Diagrams and Stratigraphy (after Mohrig et al., 2000 and Hajek, personal file) Figure 2.3: Progradational System Splay Evolution (after Perez-Arlucea and Smith, 1999) Figure 2.4: Stratigraphically Transitional and Abrupt Representations (after Jones, 2007) Figure 3.1: Location Map for Modern Systems Figure 3.2: Location Map for Wyoming Field Sites (after Jones and Hajek, 2007) Figure 4.1: Upper River Study Reach with Mapped Splays Figure 4.2: Splays Mapped near Castledale Figure 4.3: Comparison of Main Channel and Castledale Reaches Figure 4.4: Saskatchewan River Study Reach with Mapped Splays Figure 4.5: Sandover River and Ovens River Study Reaches with Mapped Splays Figure 4.6: Splay Measurements Made within Google Earth Figure 4.7. Splay Examples from the Upper River Figure 4.8: Splay Examples from the Saskatchewan River Figure 4.9: Splay Examples from the Sandover River Figure 4.10: Channel Development and Floodplain Incisions along the Ovens River Figure 4.11: Splay Areas within the Three Systems Figure 4.12: Channel Widths within the Three Systems Figure 4.13: Orthogonal Extent into Flood-basin within the Three Systems Figure 4.14: Measured Splay Widths within the Three Systems Figure 4.15: Splay Channel Path Lengths within the Three Systems Figure 4.16: Normalized Orthogonal Extent (by Channel Width) Figure 4.17: Aspect Ratio (Maximum Splay Width/Splay Channel Path Length) Figure 4.18: Splay Area vs. Channel Width in the Upper System Figure 4.19: Splay Area vs.channel Width in the Sanodver and Saskatchewan Systems vii

8 Figure 4.20: Orthogonal Extent vs. Channel Width in the Three Systems Figure 4.21: Splay Channel Path Length vs. Maximum Splay Width in the Three Systems Figure 5.1: Representation of Field Sampling Figure 5.2: Range of Visually Estimated Grain-Sizes for Facies Types Figure 5.3: Example Ferris Grain-Size Distribution Figure 5.4: Example Fort Union Grain-Size Distribution Figure 5.5: Example Willwood Grain-Size Distribution Figure 6.1: Abbreviated modeling flow plan Figure 6.2: Modeled grid and bathymetry Figure 6.3: Proportions of Each Sediment Fraction in Model Runs Figure 6.4: Cumulative Sedimentation in Channel with No Crevasse Figure 6.5: Initial Flow Magnitudes for Dry Floodplain Models Figure 6.6: Initial Flow Magnitudes for Wet Floodplain Models Figure 6.7: Crevasse and Downstream Discharges for Run Figure 6.8: Final Flow Magnitudes (Day 162) for Run 1: Fine Channel Sediment with Dry Floodplain. 107 Figure 6.9: Cumulative Erosion/Sedimentation (Day 162) for Run 1: Fine Channel Sediment with Dry Floodplain Figure 6.10: Run 1: Cross-sections A, B, and C Figure 6.11: Run 1: Cross-sections D, E, and F Figure 6.12: Cumulative Total Sediment Transport Through Crevasse Figure 6.13: Instantaneous Total Sediment Discharge Through Crevasse Figure 6.14: Normalized Instantaneous Sediment Discharge Through Crevasse Figure 6.15: Crevasse and Downstream Discharges for Run Figure 6.16: Final Flow Magnitudes (Day 153.5) for Run 2: Fine Channel Sediment with Wet Floodplain Figure 6.17: Cumulative Erosion/Sedimentation (Day 153.5) for Run 2: Fine Channel Sediment with Wet Floodplain Figure 6.18: Run 2: Cross-sections A, B, and C Figure 6.19: Run 2: Cross-sections D, E, and F Figure 6.20: Crevasse and Downstream Discharges for Run Figure 6.21: Final Flow Magnitudes (Day 198) for Run 3: Intermediate Channel Sediment with Dry Floodplain Figure 6.22: Cumulative Erosion/Sedimentation (Day 198) for Run 3: Intermediate Channel Sediment with Dry Floodplain viii

9 Figure 6.23: Run 3: Cross-sections A,B, and C Figure 6.24: Run 3: Cross-sections D, E, and F Figure 6.25: Crevasse and Downstream Discharges for Run Figure 6.26: Final Flow Magnitudes (Day 148) for Run 4: Intermediate Channel Sediment with Wet Floodplain Figure 6.27: Cumulative Erosion/Sedimentation (Day 148) for Run 4: Intermediate Channel Sediment with Wet Floodplain Figure 6.28: Run 4: Cross-sections A, B, and C Figure 6.29: Run 4: Cross-sections D, E, and F Figure 6.30: Crevasse and Downstream Discharges for Run Figure 6.31: Final Flow Magnitudes (Day 196) for Run 5: Coarse Channel Sediment with Dry Floodplain Figure 6.32: Cumulative Erosion/Sedimentation (Day 196) for Run 5: Coarse Channel Sediment with Dry Floodplain Figure 6.33: Run 5: Cross-sections A, B, and C Figure 6.34: Run 5: Cross-sections D, E, and F Figure 6.35: Crevasse and Downstream Discharges for Run Figure 6.36: Final Flow Magnitudes (Day 141.5) for Run 6: Coarse Channel Sediment with Wet Floodplain Figure 6.37: Cumulative Erosion/Sedimentation (Day 75) for Run 6: Coarse Channel Sediment with Wet Floodplain Figure 6.38: Run 6: Cross-sections A, B, and C Figure 6.39: Run 6: Cross-sections D, E, and F ix

10 List of Tables Table 1.1: Splays Described within Literature Table 2.1: Sediment Transport Predicted by Rouse Number Table 2.2: Well Described Incisionally Avulsive Systems Table 4.1: Measured Splay Quantities in Modern Systems Table 4.2: Summary Statistics and T-test Comparison Results Table 4.3. Estimated Splay Volumes and Normalized Splay Discharge Table 6.1: Grid and Bathymetry Conditions Table 6.2: Modeled Sediment Fractions Table 6.3: Vertical Sediment Thicknesses of Modeled Grain-size Distributions Table 6.4: Final Model Parameters, Initial Conditions, and Boundary Conditions Table 6.5: Surface Areas and Volumes for Splays Produced within Models x

11 Acknowledgements This thesis would not have been possible without the support of various grants to me and my adviser, Elizabeth Liz A. Hajek. I received graduate student grants from the Geological Society of America and the American Association of Petroleum Geologists, both in the amount of $2000. Additionally, this research is part of a larger grant from the National Science Foundation (Award # ) provided to Liz Hajek and Douglas Edmonds (Indiana University) for the investigation of incisional avulsion processes. Within the Penn State community, I am also personally indebted for the financial support provided by the Department of Geosciences, by my adviser, and by Chris Marone. The Department of Geosciences provided financial support and tuition for four semesters of coursework and research. Funding for summer research was provided through Chris Marone (summer 2011) and Liz Hajek (summer 2012). I am especially grateful for the support of Chris Marone, given that I am not his student and the only thing he asked for in return was that I do a good job for Liz. Beyond financial support, I appreciate the advice, suggestions, and mentoring provided by Liz Hajek. I further appreciate advice and suggestions provided by the members of my committee: Rudy Slingerland, Eric Kirby, and Doug Edmonds (an unofficial member). I am especially grateful for the access to equipment and the Delft3D license provided by Rudy Slingerland, without which, significant portions of this thesis would not have been possible. Finally, I d like to thank my family and friends for their words of encouragement and for providing the necessary amount of levity to complete this thesis. Whew, I ve made it. xi

12 Dedication During my time at Penn State, I lost several people close to my family and me. These include a family friend, Paul Kahler ( ), my granduncle and last surviving relative of my paternal grandfather, Paul Millard ( ), my grandaunt and last surviving sibling of my maternal grandfather, Mary Barzanti ( ), and my uncle, Edward Millard ( ). This thesis is dedicated to their memory. xii

13 1.0 Introduction In fluvial systems, crevasse-splay deposits develop through breaks in levees and channel banks as flow expansion outside of the channel results in decreased competence for further transport of sediment, which leads to overbank deposition (Bridge and Demicco, 2008). Crevasse splays play an important role in levee development, floodplain sedimentation, and channel avulsion processes. Crevasse-splay deposits generally consist of clay, silt, and sand, with coarser sediments deposited proximal to the channel and finer sediments deposited farther from the channel (e.g. Smith et al., 1989) (Figure 1.1); consequently, crevasse-splay deposition also has important consequences for the size and inter-connectedness of hydrocarbon reservoirs and groundwater aquifers located within buried fluvial systems. Crevasse-splays have been studied extensively in several modern and sub-surface Holocene systems (e.g. Smith et al., 1998; Bristow et al., 1999; Perez-Arlucea and Smith, 1999; Stouthamer, 2001), and in ancient systems through outcrop (e.g. Eberth and Miall, 1991; Mjos et al., 1993; Sarti et al., 2001; Pranter et al., 2009) and subsurface studies (Reynolds, 1999), as presented in Table 1.1. Nevertheless, it remains difficult to predict crevasse-splay characteristics in a fluvial system with given boundary conditions. In this study, I aim to determine which variables exert primary control on crevasse-splay size for similarly sized fluvial systems. While several variables influence crevasse-splay development and growth, I hypothesize that factors controlling sediment supply and sediment-dispersal patterns should control the characteristic size and volume of crevasse splays produced by a given fluvial system. Sediment supply is important because very fine sediments may not be deposited proximal to channel margins and very coarse sediments transported as bed load may be unlikely to leave the channel through crevasses. Additionally, excessive supply may promote quick crevasse healing and limited supply may promote further splay growth. On the floodplain, flow patterns determine where and how sediment is dispersed, which influences the extent of crevasse-splay deposits. For these reasons, I hypothesize that splay size and volume is most dependent on variables that influence floodplain sediment supply and deposition patterns. Within fluvial systems there are three factors ceteris paribus that impact both sediment supply to and deposition patterns on the floodplain: (1) river discharge, (2) floodplain water-surface slope, and (3) channel sediment grain-size distribution. River discharge is the most 1

14 direct control on crevasse-splay growth, as higher discharges lead to a higher sediment supply and greater potential for extensive overbank sediment dispersal. Thus, river discharge should inherently set the maximum potential crevasse-splay scale within a given system, as high water and sediment discharges lead to larger crevasse-splay deposits. Floodplain water-surface slope influences spatial deposition rates away from the crevasse opening (e.g. Pizzuto, 1987; Adams et al., 2004; Hajek and Wolinsky, 2012) and different water-surface gradients can lead to different water (and sediment) discharges through crevasses. Channel sediment grain-size distribution influences both the availability of material to the splay (e.g. Slingerland and Smith, 1998) and downstream sediment extraction (i.e. deposition) profiles (e.g. Parker, 1991; Cazanacli and Smith, 1998; Fedele and Paola, 2007). Review of previously studied crevasse-splays (Table 1.1, Figures ) shows that, although large rivers can produce large crevasse splays, it is not necessarily the dominant control on crevasse-splay size in different systems. For example, large crevasse splays are formed via both large-scale rivers (e.g. glacial outwash Mississippi River, Blum et al., 2000) and relatively small rivers (e.g. Texas s Colorado River, Aslan and Blum, 1999), and there is no predictable relationship between splay dimensions (i.e. length, width, and thickness) and main channel width (a rough proxy for discharge), as shown in Figures The relationship between channel and splay sizes may be particularly weak because, in this data set, channel width is weakly correlated to discharge (e.g. the Niobrara River has a channel width of ~250 meters, but an average discharge of only 48 m 3 /s, Bristow et al., 1999). Nonetheless, based on these data, channel scale cannot be used as a direct predictor of crevasse-splay scale. Figure 1.5 provides an example of this, where, although the Amazon River is substantially larger than the Saskatchewan River, the scale of crevasse splays in the Saskatchewan system are large relative to the size of the main river, while Amazon splays are small relative to the scale of the main channel. Thus, for a channel with a given discharge, crevasse-splay size may be largely influenced by factors that affect the downstream mass-balance within the crevasse-splay system: floodplain water-surface slopes and channel grain-sizes. These factors specifically control the spatial deposition and downstream fining rates from the crevasse mouth to the distal splay edge. To evaluate how floodplain water-surface slope and channel grain size affect splay development, I tested two main hypotheses: (1) crevasse-splay size and volume is larger in systems featuring steep cross-floodplain water-surface gradients that allow for the development 2

15 of advective currents to draw sediment away from the channel margin and into the floodbasin (e.g. Adams et al., 2004), and (2) crevasse-splay size and volume is greater in systems featuring intermediate grain-size distributions (i.e. coarse silt to fine sand in sand-bed rivers) that is both easily suspended in channels and readily deposited in unconfined overbank flows. To test these hypotheses, I employed a multifaceted approach that included aerial-image analysis of modern rivers using Google Earth, field measurements in ancient systems, and morphodynamic modeling of splay development using Delft3D software. The following sections detail background and the results for each study. Combined results of these studies suggests that floodplain water-surface slope and channel grain-size are useful predictors of crevasse-splay abundance and size in modern and ancient systems, and that, ultimately, floodplain water-surface slope may be the primary control on whether large splays form in alluvial systems. 3

16 Figure 1.1: Splay Development Schematic. (A) A currently active splay with sands deposited proximally and mud deposited distally. In this system, numerous abandoned splays are evident and have contributed to floodplain and levee aggradation along the channel. (B) When beyond threshold conditions (e.g. Slingerland and Smith, 1998), splay evolution may lead to avulsion for a channel system. 4

17 Location/Formation (Source) Umbum Creek terminal splay, (dry) Lake Eyre, central Australia (Lang et al., 2004) Southern splay, Wild Cow Island, Colorado River, north of Matagorda, Texas (Aslan and Blum, 1999) Northern splay, Colorado River, north of Matagorda, Texas (Aslan and Blum, 1999) Windy Lake Splay (ca AD), Mossy River, Cumberland Marshes, Saskatchewan, Canada (Smith and Perez-Arlucea, 1994) Splay off Brant Bayou ( AD), Mississippi River delta, LA (Roberts, 1997) Douglas Creek terminal splay, (dry) Lake Eyre, central Australia (Fisher et al., 2008) East Splay, Niobrara River near confluence with Missouri, Nebraska, May June 1994 (Bristow et al., 1999) Classification Type Splay Length (meters) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Table 1.1: Splays Described within Literature. Splay Width (meters) Splay Height (meters) Splay Grain-Sizes Single Splay Mostly finegrained sand, some coarse sand near mouth, some silts and muds distally Single Splay Single Splay Single Splay (mean ~2) 1 2 Very Fine Fine, silt, based on similar subsurface deposits (see below) 1 2 Very Fine Fine, silt, based on similar subsurface deposits Very Fine Fine Sand, silt, clay Single Splay Up to Very Fine Medium sand Single Splay Medium Coarse sand proximally, Very Fine sand, silt, and clay Single Splay Medium sand with fine-grained sands and silts downdip Channel Notes Width (meters) 200 Dimensions from Google Earth, channel depth 2-3 meters 125 Channel width measured in Google Earth 100 Channel width measured in Google Earth Channel width from Google Earth 150 Channel depth meters 250 Channel discharge is approximately 48 m 3 /s 5

18 Location/Formation (Source) West Splay, Niobrara River near confluence with Missouri, Nebraska, autumn August 1996 (Bristow et al., 1999) Colville River Delta, North Slope, Alaska (Tye, 2004) Nanuk Lake splay, began pre-1949, Colville River Delta, North Slope, Alaska (Tye, 2004) Recent ( AD) splay north of Centre Angling Channel, Cumberland Marshes, Saskatchewan, (Farrell, 2001) Corps Breach intentional levee breach ( AD), Cosumnes River, California (Florsheim and Mount, 2002) Accidental Forest intentional levee breach ( AD), Cosumnes River, California (Florsheim and Mount, 2002) Classification Type Splay Length (meters) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Modern (Surface) Splay Width (meters) Splay Height (meters) Splay Grain-Sizes Single Splay Medium sand with fine-grained sands and silts downdip Composite (n = 14) (mean = 630) (mean = 380) Not reported Very Fine to Fine sand (based on Nanuk Lake Splay) Single Splay Not reported Very Fine to Fine sand Single Splay Silt to Fine sand, with Very Fine Medium sand in splay-channel Single Splay 428 (max) Single Splay 387 (max) 195 (max) 207 (max) % is Medium and Coarse sand % is Medium and Coarse sand Channel Notes Width (meters) 250 Channel discharge is approximately 48 m 3 /s (40 channels) Channel depths up to 13 meters, discharge of 492 m 3 /s (probably combined) 200 Channel depths up to 12 meters 100 Channel depth meters, discharge is approximately 450 m 3 /s m 3 /s to cause flooding, Channel width from Google Earth m 3 /s to cause flooding, > 100 m 3 /s to cause major extraction, Channel width from Google Earth 6

19 Location/Formation (Source) 1782 AD flood of Thinhope Burn, Northern Pennines, UK (Macklin et al., 1992) Ephemeral tributary of Clarence River, northeast New South Wales, Australia (O Brien and Wells, 1986) Inland deltaic splay ( AD), Rhine-Meuse delta, Netherlands (Kleinhans et al., 2010) Holocene, lower Mississippi River Valley, Missouri - Arkansas (Blum et al., 2000) Sub-surface splays, Colorado River, near Matagorda, Texas (Aslan and Blum, 1999) Holocene (ca ybp), Lower Mississippi River Valley, Tensas Basin, Nolan Site, Indian Mound Crevasse, northeast Louisiana (Arco et al., 2006) Lopik splay complex (~ years before present (ybp)), Rhine-Meuse Delta, Netherlands (Stouthamer, 2001) Zuid-Stuivenberg splay complex (ceased ~3660 ybp), Rhine-Meuse Delta, Netherlands (Stouthamer, 2001) Classification Type Splay Length (meters) Modern (Surface) Modern (Surface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Splay Width (meters) Splay Height (meters) Splay Grain-Sizes Single Splay < 1 Cobbles and boulders with sandy matrix, some bedded sand Single Splay Medium Coarse moderately sorted sand Single Splay Fine Medium fining upward, silt and clay Single Splay Medium Coarse grading to silt and clayey silt down valley Composite 4000 Not reported 1 2 Very Fine Fine, silt Single Splay < 1.5 Fine sand and silt, coarsening upward Single Splay Single Splay Silty clay, sandy clay, Fine Coarse sand (4-5 in some splay channels) Silty clay, Sandy clay, F-M sand (mostly silty clay downstream) Channel Width (meters) 10 Notes 5 6 Channel depth approximately 1 meter Not reported Not reported Not reported (channel belt) (channel belt) Channel depths from approximately 5 15 meters Channel depths from approximately 5 15 meters 7

20 Location/Formation (Source) Western Hallandse Ijssel Splay (channel active ybp), Rhine-Meuse Delta, Netherlands (Stouthamer, 2001) Eastern Hallandse Ijssel Splay (channel active ybp), Rhine-Meuse Delta, Netherlands (Stouthamer, 2001) Breached dyke at Diefdijk, Netherlands, 1571 AD (Hesselink et al., 2003) Westernmost Holocene subsurface splay, Cabauw area, Rhine-Meuse delta, Netherlands (Weerts and Bierkens, 1993) 2 nd easternmost Holocene splay, Cabauw area, Rhine- Meuse delta, Netherlands (Weerts and Bierkens, 1993) 2 nd westernmost Holocene splay, Cabauw area, Rhine- Meuse delta, Netherlands (Weerts and Bierkens, 1993) Easternmost Holocene splay, Cabauw area, Rhine-Meuse delta, Netherlands (Weerts and Bierkens, 1993) Holocene (<5000 ybp), Lower Mississippi River Valley, near Ferriday, Louisiana (Aslan and Autin, 1999) Classification Type Splay Length (meters) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Recent (Subsurface) Splay Width (meters) Single Splay Splay Splay Grain-Sizes Height (meters) Silty clay, Sandy clay, F-M sand (mostly silty clay downstream) Single Splay Silty clay, Sandy clay, F-M sand (mostly silty clay downstream) Single Splay Sandy to silty clay, sand ( um) in splaychannels Single Splay Up to 4 Fine sand, sandy clay, silty clay Single Splay Fine sand, sandy clay, silty clay Single Splay Up to 3 Fine sand, sandy clay, silty clay Single Splay Fine sand, sandy clay, silty clay Composite Very Fine to Medium sand, clay and silt Channel Width (meters) (channel belt) (channel belt) (channel belt) Notes Channel depths from approximately 5 15 meters Channel depths from approximately 5 15 meters Channel depths meters 8

21 Location/Formation (Source) Various paralic sandstones, no explicit locations given (Reynolds, 1999) Upper Permian Tangal Coal Measures, Bowen Basin, Australia (Michaelson et al., 2000) Upper Triassic, Callide Seam Member, Queensland, Australia (Jorgensen and Fielding, 2008) Late Pennsylvanian to Early Permian, Cutler Formation, north-central New Mexico (Eberth and Miall, 1991) Late Permian, Beaufort Group, Karoo Sequence, South Africa (Smith, 1993) Upper Cretaceous, lower Williams Fork Formation, Coal Canyon, Colorado (Pranter et al., 2009) Middle Devonian Gauja Formation, Baltic Basin, southern Estonia to southern Lithuania (Ponten and Plink- Bjorklund, 2007) Classification Type Splay Length (meters) Ancient Composite (n = 84) from multiple locations (mean = 5577) Splay Width (meters) (mean = 787) Ancient Composite 5000 Not reported Ancient Composite 4000 (max) Ancient Composite 100s 1000s Splay Height (meters) (mean = 1.4 m) (presumably multiple sequences) Splay Grain-Sizes Channel Width (meters) Notes Not reported Channel depths from 1-40 meters Fine Medium in splay-channel (5.2 m thick), fine sandstone and siltstone 2500 Less than 4 Fine Coarse sandstones and sandy siltstones with coaly traces < 1000 < 10 (stacked deposits) Silty to pebbly sandstone, fines upward Ancient Composite < 2 Fine-grained sandstones and mudstones Ancient Composite (n = 279) Not reported feet (median = feet) feet (median = 4.7 feet) Very Fine to Fine sand Ancient Single Splay Very Fine to Medium sand (bimodal), grades into mudstone 1000 Channel depth ~ 15 meters for single story 2800 (channel belt) Channel depths less than 10 meters < 1000 Channel depths 2 5 meters Not reported feet (median feet) Not reported Channel body thicknesses of feet (median 11.8 feet) 9

22 Figure 1.2: Splay Length vs. Channel Width. This figure consists of values compiled from literature and provided within Table 1.1. This chart excludes large composite data sets and systems where only the channel-belt width is presented. The values for the Amazon splays shown in Figure 1.5 are provided. Saskatchewan splays are highlighted (not specifically shown in Figure 1.5). 10

23 Figure 1.3: Splay Width vs. Channel Width. This figure consists of values compiled from literature and provided within Table 1.1. This chart excludes large composite data sets and systems where only the channel-belt width is presented. The values for the Amazon splays shown in Figure 1.5 are provided. Saskatchewan splays are highlighted (not specifically shown in Figure 1.5). 11

24 Figure 1.4: Splay-Deposit Thickness vs. Channel Width. This figure consists of values compiled from literature and provided within Table 1.1. This chart excludes large composite data sets and systems where only the channel-belt width is presented. Saskatchewan splays are highlighted (not specifically shown in Figure 1.5). 12

25 Figure 1.5: Comparison of Saskatchewan and Amazon Splays. (A) The Saskatchewan River system features splays that are large in size relative to channel width. Additionally, these splays feature a more distributary planform with multiple bifurcating channels. (B) The Amazon River system features splays that are small in relation to the channel scale and tend to form a single dominant finger-like channel prograding into a lake. (Images are from Google Earth and copyrights are held by Cnes/Spot Image (A), GeoEye (B), U.S. Geological Survey (B), and Digital Globe (B).) 13

26 2.0 Background Crevasse splays have been described in a range of modern, Holocene, and ancient fluvial systems (Table 1.1). Splays are found in a wide variety of fluvial systems ranging from small tributaries (e.g. Cosumnes River, California, Florsheim and Mount, 2002) to the glacial outwash Mississippi (Blum et al., 2000), humid (e.g. Texas s Colorado River, Blum et al., 2000) to arid (e.g. terminal splays in Lake Eyre, Australia, Lang et al., 2004) environments, and with grain-sizes ranging from clays (e.g. Rhine-Meuse Delta, Netherlands, Stouthamer, 2001; Hesselink et al., 2003) to cobbles (e.g. Northern Pennines, United Kingdom, Macklin et al., 1992). Ultimately, it is the interplay of these and other variables that controls splay development. Here, I review how different variables influence crevasse-splay development. 2.1 Controls on Splay Development As overbank depositional products of fluvial systems, crevasse-splay development and growth is controlled by in-channel inputs, which supply the splay, and floodplain conditions, which influence how and where splay deposits accumulate. These boundary conditions will collectively determine the volume, area, and planview shape of a splay. As discussed in the Introduction, this study is primarily concerned with conditions, ceteris paribus, that cause changes in the volume and extent of splays in different systems. Thus, conditions which primarily exert control over planview shape, but have little influence on sediment supply through levee crevasses, are not considered primary controls on splay volume and mass balance. Many factors can influence the shape and specific planform geometry of crevasse-splay deposits. For example, vegetation and floodplain cohesion may play a pivotal role in some systems, but these variables will primarily manifest as modifications in sediment-routing and dispersal patterns across floodplains. For example, vegetation may increase the effective cohesiveness of floodplain deposits (e.g. Saskatchewan River, Smith et al., 1989) and/or act as a baffle creating low overbank-flow velocities and potentially low water-surface gradients (e.g. Okavango Fan, McCarthy et al., 1992; Smith et al., 1997). In some systems, the presence of large animals, such as hippopotamus or elk, create floodplain pathways and/or depressions, which leads to increased crevasse discharge and overbank-flow routing (e.g. Okavango Fan, McCarthy et al., 1992; Narew River, 14

27 Gradzinski et al., 2003; Klip River, Tooth et al., 2007). While these conditions are important for overall splay development, they primarily affect the shape of crevasse-splay deposits; this study focuses specifically on splay size rather than form Primary Controls on Splay Area and Volume Here, I consider three conditions as primary controls on the sediment supply available for splay growth: (1) channel (and associated crevasse) discharge, (2) floodplain water-surface slope, and (3) channel grain size. While channel discharge is important in determining overall splay volume and area, within a given system, or between systems of similar scale, differences in splay size may be controlled largely by cross-floodplain water-surface gradient and the channel grain-size distribution. The importance of floodplain water-surface gradients lies in the balance between advective and diffusive transport processes, which determine the competence for floodplain sediment transport (Figure 2.1) (e.g. Pizzuto, 1987; Adams et al., 2004; Hajek and Wolinsky, 2012). Advective transport results from cross-floodplain currents capable of transporting sediment as suspended load or bedload (Adams et al., 2004), while diffusive transport results from turbulent processes that arise at the free shear boundary between rapid channelized flow and stagnant (or much slower) floodplain flow (Rajaratnam and Ahmadi, 1979). These turbulent processes produce eddies that propagate into the floodbasin and are capable of diffusing suspended sediment across the floodplain (Adams et al., 2004). In systems where the floodplain water-surface elevation is below that of the channel, the difference in these water-surface elevations results in a cross-floodplain water-surface gradient (Figure 2.1A). Steeper gradients lead to higher velocity advective currents (e.g. Adams et al., 2004; Hajek and Wolinsky, 2012). Thus, cross-floodplain advective-dominated transport is most common on wide and well-drained ( dry ) floodplains (Adams et al., 2004; Hajek and Wolinsky, 2012). Where strong enough, these advective currents can transport even fine sand onto the floodplain (Adams et al., 2004). While turbulent diffusive processes still occur on these floodplains, the importance of diffusive eddies is overshadowed by these cross-floodplain advective flows (Adams et al., 2004). 15

28 In laterally constricted and poorly drained ( wet ) settings (e.g. Hajek and Wolinsky, 2012), water surface on the floodplain rises in concert with channel stage, which inhibits the development of a water-surface slope away from the channel and strong currents onto the floodplain (e.g. Pizzuto, 1987; Adams et al., 2004; Hajek and Wolinsky, 2012). Instead, floodplain waters may be relatively stagnant, while channelized flow exiting a levee crevasse is much more rapid, resulting in the development of turbulent eddies at the boundary between the channelized and stagnant water, as shown in Figure 2.1B (e.g. Adams et al., 2004). These eddies can transport some transferred suspended sediment from the main channel, but the abrupt decrease in turbulence across the floodplain causes the coarser sediment to fall out of suspension within a short distance of the channel margin (Adams et al., 2004). Hajek and Wolinsky (2012) predict that this diffusive settling length is potentially around 1% of advective settling length. Given the various settings and processes (e.g. vegetation) for fluvial systems, it is likely that different floodplains exhibit different characteristic mixes of diffusive and advective transport processes (Adams et al., 2004). As proximal-overbank features, crevasse-splay deposition requires an appropriate sediment load and grain-size distribution that enables both (1) sediment transport beyond the channel s banks and (2) sediment deposition proximal to the channel during flood events. Coarse sediments, relative to a channel s competence, are transported as bedload and may never be suspended high enough in the water column to escape from the channel. Conversely, transport of excessively fine sediments as washload may continue overland until re-joining a channel or until the water reaches a ponded topographic low perhaps on the distal floodplain where fine sediments may settle out in still water. Thus, crevasse-splay growth seems to require abundant, relatively coarse suspended grains, (e.g. particles ranging from coarse silt to fine sand for sandbed rivers, i.e. slope O[10-4 ]). The Rouse (1937) number is a ratio of grain-settling velocity (a function of particle size, where larger grains generally settle faster) to shear velocity within a flow, and is determined by the equation P = ω s Ku [1] where P is the non-dimensional Rouse number, ω s is the particle settling velocity, κ is von Karman s constant (approximately 0.41), and u * is the shear velocity, which is determined at the bed of the channel through the equation 16

29 u = τ b ρ [2] where τ b is the shear stress at the lower boundary (i.e. bed/ground), and ρ is the density of water. The Rouse number is useful for describing when and how sediment of a given grain size will be transported in a given flow. When the Rouse number is large (e.g., P > 12.5), grains are too large to be transported. When Rouse number is very small (e.g., P < 0.5), particles will be transported entirely as washload (Table 2.1). As water escapes a levee it generally undergoes a sudden lateral expansion on the open floodplain that causes a drop in shear velocity and shear stress. As this happens, sediment can be transferred to the bed and ultimately deposited on the floodplain. The Rouse number is useful for considering how particle size and flow deceleration (influenced by floodplain drainage conditions) can both influence crevasse-splay deposition. 2.2 Relationship of Splays and Fluvial Avulsion Given the morphodynamic formation of splays through crevasses, these deposits are closely associated with fluvial avulsions. In three of the most thoroughly studied Holocene and modern systems, the Mississippi River (e.g. Aslan and Autin, 1999), the Saskatchewan River (e.g. Smith et al., 1989), and the Rhine-Meuse Delta (e.g. Stouthamer, 2001), crevasse-splay development is closely linked to avulsion processes. In each of these systems, a significant portion of overbank deposition is attributable to the avulsion process and the building of coeval splay deposits (e.g. Aslan and Autin, 1999; Smith and Perez-Arlucea, 1994; Makaske, 1998). These splays consist of fine-grained sediments interspersed with coarser sand beds correlating with either more proximal deposits or crevassechannels (Bridge and Demicco, 2008). Slingerland and Smith (2004) categorized fluvial avulsions into three types with two distinct end members, as originally described by Mohrig et al. (2000). The two end members are progradational and incisional avulsions (Figure 2.2). In progradational avulsions, the development of a new channel occurs when flow is diverted through a crevasse into a floodbasin, resulting in the loss of competence for sediment-laden flows and deposition of coarser suspended sediment as a splay. Progressive building of the channel leads to further progradation of this splay wedge across the floodplain in the downstream direction (Slingerland and Smith, 2004). In incisional avulsions, the development of new channel pathways progress upstream from 17

30 knickpoints through headward erosion of the floodplain. When this erosional pathway intersects the existing channel, it captures the channel s discharge resulting in an avulsion (Mohrig et al., 2000; Slingerland and Smith, 2004). The third type annexation avulsions is a subset where avulsion occurs through re-occupation of a pre-existing channel or side channel, which may result in crevasse-splay deposits when the annexed channel is insufficient to carry the full discharge (Slingerland and Smith, 2004). Annexation avulsion is documented in the progradational Saskatchewan (Smith et al., 1998; Perez-Arlucea and Smith, 1999; Slingerland and Smith, 2004) and Rhine-Meuse systems (Stouthamer, 2001), incisional Baghmati River system (Jain and Sinha, 2004), and mixed progradational/incisional lower Rio Pastaza system (Bernal et al., in press). Slingerland and Smith (2004) conclude that annexation avulsions likely make up a portion of most major avulsion sequences Splays within Progradational Avulsion Systems Splays produced during progradational avulsions take on a variety of shapes--including lobate, elliptical, and elongate--and stages of development (Smith et al., 1989; Slingerland and Smith, 2004). Within the Saskatchewan system, Smith et al. (1989) delineated the progression of development and maturity in splays into three stages. Stage I splays are small (< 1 km 2 ) lobate splays deposited by sheet flow or poorly defined channels. Stage I splays progressively develop into Stage II splays, which are larger (up to 8 km 2 ), have mature channels, and become interconnected with adjoining splays. Stage III splays are even larger (up to 20 km 2 ), generally transport finer sediments than Stages I and II, and make up the distal, elongate extensions of Stage I and II splays that have combined to form a single large anastomosed unit (Figure 2.3). Smith and Perez-Arlucea (1994) estimated that 50-70% of the Saskatchewan avulsion belt consists of these Stage III splays. Similar development of avulsion-related crevasse-splays is observed in post-glacial Rhine-Meuse Delta deposits (Makaske, 1998; Stouthamer, 2001), in Holocene Mississippi River deposits (Aslan and Autin, 1999), and in Holocene Colorado River (Texas Gulf Coast) deposits (Aslan and Blum, 1999). In general, recent and modern progradational systems show extensive development of avulsion-related splays that fill a significant portion of the flood-basin within these systems (e.g. Smith and Perez-Arlucea, 1994; Makaske, 1998). 18

31 2.2.2 Splays within Incisional Avulsion Systems Overbank deposition including splays is not an innate process of initial channel development during incisional avulsion, which contrasts with progradational avulsions (Mohrig et al., 2000; Slingerland and Smith, 2004). Rather, incisional avulsions likely occur in systems where floodplain erosion outpaces channel super-elevation and/or increases in cross-valley gradient. Given the close relationship between aggradation and the potential to develop splays (e.g. Bridge and Demicco, 2008), incisional avulsions are often presumed common in systems with little or no net aggradation (Slingerland and Smith, 2004). Notably, incisional avulsions are not restricted to degradational settings (Table 2.2), and net-aggradational systems avulsing by incision often lack crevasse-splay deposits. For example, the Narew River (Poland) is rapidly aggrading (~1-1.5 mm/y), but splays and associated overbank deposits (e.g. levees and alluvial ridges) are not observed (Gradzinski et al, 2003). In contrast, splays are present within the rapidly aggrading Baghmati River system (India) (Jain and Sinha, 2004) and the Magela Creek system (Australia) (Tooth et al., 2008), both of which avulse by incision. The slowly aggrading Channel Country (Australia) features no along-channel splays, but does feature splay-like deposits at waterholes representing downstream terminal fans of channelized overland floodways (Gibling et al., 1998). However, splay-like features are observed near the initiation points of overland flow paths in the slowly aggrading, fine-grained (primarily clay and silt) Red Creek system (Wyoming, USA) (Schumann, 1989). In other low or non-aggradational settings, related overbank deposits such as, levees and/or alluvial ridges are present in the Ovens and Kings Rivers (Australia) (Schumm et al., 1996), Fitzroy River (Australia) (Taylor, 1999), and Klip River (South Africa) (Tooth et al., 2007), although splays are not specifically described in any of these systems and are probably absent from (at least) the latter two. Within incisional systems, the best documentation of splay development and overbank sedimentation is within the Baghmati River system and this system differs markedly from progradational systems described in the previous section (2.2.1). One Baghmati splay deposited within a single flood season measured 200 meters in length and 100 meters in width for a channel of approximate bankfull width of 100 meters (Sinha et al., 2005), which is small in comparison to splays produced in systems of similar size (e.g. Table 1.1). Despite frequent crevassing, there is little evidence of progradational splay complexes (Sinha et al., 2005), and 19

32 distal floodplain deposits consisting of interbedded fine-grained sand and shelly clay were found only 200 meters from the main active channel (Sinha et al., 2005). Based on the small scale of splays and rapid accumulation of distal floodplain deposits, Sinha et al. (2005) argue that basin-filling in the system is primarily through "repeated flood deposits" rather than avulsionassociated deposition. 2.3 Crevasse-splays in the Stratigraphic Record As a primary product of many fluvial avulsions, crevasse-splay abundance and characteristics may be key to understanding flood-basin filling processes within ancient fluvial successions. Jones and Hajek (2007) identified two types of fluvial stratigraphy, which they termed stratigraphically transitional and stratigraphically abrupt. Transitional successions consist of abundant splays, and other proximal overbank deposits beneath and lateral to paleochannel deposits, while abrupt successions feature paleochannel bodies directly overlying and encased by fine-grained distal floodplain deposits (Figure 2.4). Jones and Hajek (2007) also note that ancient systems dominated by transitional stratigraphy have abundant and extensive crevassesplay deposits, while similar deposits are rare-to-absent in ancient deposits dominated by abrupt stratigraphy. These differences in splay proneness and extent suggest the primary controls for splay growth and development may be determined from ancient formations. Stratigraphically-transitional deposits feature paleochannel deposits underlain by coarsening-upward successions that include splay deposits (Jones and Hajek, 2007). Examples are found in the Paleocene Fort Union and Paleocene-Eocene Willwood formations of the Bighorn Basin, Wyoming (Jones and Hajek, 2007). In these Formations, Kraus and Aslan (1993), Kraus (1996), and Kraus (1998) identified several-meter thick, several-kilometer wide lithologically heterogeneous horizons with little or no paleosol development, which suggests rapid floodplain deposition. Ultimately, these authors linked these heterolithic horizons with the Stage III splays and avulsion sequences of Smith et al. (1989). Kraus and Wells (1999) further examined this relationship and determined these heterolithic avulsion deposits consist of 60% clay on average and feature no or limited erosion by or into the encasing well-developed, finegrained paleosol horizons. Heterolithic horizons make up 59 and 65% of Fort Union Formation and Willwood Formation stratigraphic sections, respectively, and feature coarsening-upward 20

33 successions of claystone/mudstone to siltstone/sandstone sheets, and are always overlain by a major sheet sandstone paleochannel body (Kraus and Wells, 1999). In comparison to the Saskatchewan and Rhine-Meuse systems, the ancient formations have similar coarsening-upward successions overlain by channels (e.g. Smith et al., 1989; Perez-Arlucea and Smith, 1999), ribbon channel (i.e. Stage III) anastomosed patterns (e.g. Smith et al., 1989; Stouthamer, 2001), predominance of very fine-grained facies (e.g. Smith et al., 1989; Stouthamer, 2001) and avulsion-related deposits (e.g. Smith and Perez-Arlucea, 1994; Makaske, 1998). Kraus and Wells (1999) also discuss the similarity of the ancient deposits with other systems including progradationally linked Holocene deposits of the Colorado River in Texas (Aslan and Blum, 1999) and the Miocene Chinji Formation of Pakistan (Willis and Behrensmeyer, 1994). In contrast, stratigraphically abrupt formations feature coarse-grained paleochannel deposits positioned directly above floodplain deposits with no or little evidence of avulsionrelated coarsening-upward successions (Jones and Hajek, 2007). Jones and Hajek (2007) identified an example in the Upper Cretaceous-Paleocene Ferris Formation (Hanna Basin, Wyoming), and similar successions were described by Mohrig et al. (2000) in the Oligocene Guadalope-Matarranya system (Spain) and the Eocene Shire Member of the Wasatch Formation (western Colorado). The lack of avulsion-related deposits means these formations either fully reworked these deposits or did not feature extensive crevasse-splay deposition during avulsion (Jones and Hajek, 2007). The latter possibility suggests these deposits formed in systems that primarily aggrade via overbank flood sedimentation (Jones and Hajek, 2007), and possibly in incisional avulsion systems (Mohrig et al., 2000; Jones and Hajek, 2007). 21

34 Figure 2.1: Floodplain Drainage Types (after Adams et al., 2004). (A) On well-drained floodplains, advective-sediment transport dominates as significant water-surface gradients (β) develop due to differences in channel and overbank water-surface levels, leading to advective currents away from the channel (arrows) capable of transporting sediment. This is common on laterally wide and dry floodplains (Hajek and Wolinsky, 2012). (B) On floodplains with standing water, diffusive-sediment transport occurs as nearly equal water surface levels between the channel and distal floodplain prevent the development of advective currents away from the channel. Instead, diffusive eddies arise parallel to the channel (as depicted by the spirals) due to differences in flow velocity between the channel and floodplain, which allows for limited transport of sediment through turbulence. This is more common on laterally narrow and wet floodplains (Hajek and Wolinsky, 2012). 22

35 Table 2.1: Sediment Transport Predicted by Rouse Number. The Rouse (1937) number is useful in predicting the sediment transport mode for a given grainsize and shear-velocity (Values from Shah-Fairbank et al., 2011). Rouse Number Range Sediment Particle Transport P 12.5 Little or no movement 12.5 P 5.0 Bedload 5.0 P 1.25 Suspended significant contact with bed (mixed load) 1.25 P 0.5 Suspended Less than 20% in contact with bed 0.5 P Suspended 100% wash-load 23

36 Figure 2.2: Progradational and Incisional Avulsion Diagrams and Stratigraphy (after Mohrig et al., 2000 and Hajek, personal file). (A) Progradational avulsions feature sediment-laden flow through crevasse(s) building prograding splay wedges (top, in brown) via anastomosed pathways (top). Eventually, these pathways coalesce into a new primary channel (middle) that captures the main channel s flow. The nascent channel may rejoin this channel downstream (as shown via tributary) or cut an entirely different pathway and abandon the previous course. Stratigraphically, this produces interbedded and coarsening-upward sequences of sediment from the splay wedge (tan in bottom panel) correlating with the heterolithic avulsion deposits of Kraus and Wells (1999). (B) Early stage incisional avulsions (top) consist of overbank flows eroding floodplain pathways, and may follow previously developed topographic lows (brown). These flows may re-enter the main channel farther downstream at an erosional knickpoint (as shown) or cut a new pathway via topographic low. Over time overbank flow will cause headward erosion from the knickpoint/topographic low that eventually intersects the preceding trunk channel causing an avulsion. In stratigraphic cross-section this channel may consist of abrupt deposits (Jones and Hajek, 2007) where the channel sandstone and levee wings (yellow) would overlay floodplain deposits (purple and red). 24

37 Figure 2.3: Progradational System Splay Evolution (after Perez-Arlucea and Smith, 1999). (A) Crevasse(s) in main channel levee leads to nascent development of Stage I splay sand sheet via numerous distributary channels. (B) Primary conduits develop to transport bulk of overbank flows and these conduits begin to develop levees and an anastomosing pattern representative of Stage II splays. (C) Further anastomosis and elongate extension of distal mouth bars is representative of Stage III splays. 25

38 System Red Creek, Wyoming, USA (Schumann, 1989) Small tributaries of the Ohio River, Indiana, USA (Miller, 1991) Okavango Fan, Botswana (McCarthy et al., 1992) (Smith et al., 1997) Ovens and King Rivers, Victoria, Australia (Schumm et al., 1996) Channel Country, eastcentral Australia (Gibling et al., 1998) Fitzroy River, Western Australia, Australia (Taylor, 1999) Avulsion Mechanism Loss of channel capacity due to in-channel benches (p ) Loss of channel capacity due to in-channel accretion (p. 226) Loss of channel capacity due to in-channel accretion via excessive bedload and vegetation (p. 781, 791) Loss of channel capacity due to in-channel accretion resulting from extensional faulting (p ) Loss of channel competence due to increased sinuosity and inchannel accretion (p ) Loss of channel capacity due to in-channel benches restricting flow capacity (p. 610) Channel insufficient to carry high discharge floods (tropical cyclones and large monsoonal storms) and high velocity overbank flows (p. 85) Table 2.2: Well Described Incisionally Avulsive Systems. Splays and other Grain Sizes Floodplain Deposition Character Small splaylike deposits near overbank flow initiation point (p.284) Apparently none and overbank flows are devoid of coarse grains (p. 226) Splays and related deposition is absent (p. 781), no levees form due to lack of silt and clay (p. 791), rather form from vegetation and peat (p. 793) Lacks splays, levees, banks, and mud-rich flood-basins (p. 51) No mention of splays, alluvial ridges and discontinuous levees are present (p. 1217), channels aggrade for short periods of time (p. 1223) Splays not observed, although small splaylike structures found at waterholes (p. 602), small and wide levees (p ), pelleted muds breakdown and are deposited on floodplain (p. 604) Splays are likely rare or absent (p. 85), levees are intermittent or absent and topographically subtle (p. 81) 62-99% silt and clay with median of 3 microns (p ) Gravel alluvium with fine to coarse sand matrix in channels (p. 223) Fine sand for bedload (p ) and very little suspended load, bedload > suspended load (p ) Fine-medium sand with a paucity of silt and clay only 15% of load is suspended (p ) Gravel-bedded in Brookfield Reach (~50 mm) and coarse sand and small gravel bedded in Pioneer Reach (p. 1215) Clay to fine sand, with some medium to very coarse sand consisting of aggregated mud transported as bedload (p ) Not specifically mentioned, but large sand-sized bedload and clayey suspended load (p. 81) Arid climate with grasses and sagebrush (p ), gradient (p. 286) Occurs within hardwood forest (p. 222), Silt to coarse sandy loam in floodplain (p. 223) Semi-arid, but swampy with emergent aquatic vegetation (p. 779), 43% of channel flow is lost to swamps (p. 786) Swampy, consists of papyrus (sedge) and reed grasses rooted in submerged peat (p ) Floodplain inundated frequently (p. 1216), dense bank vegetation along mature channels (p. 1217) Arid to semi-arid climate prone to monsoonal weather (p. 597), vegetation is sparse (p. 598), fully covered during floods except for aeolian dunes (p. 602), low water table (p. 609) Inundation up to 3 meters in high discharge floods (~10000 m 3 /s) on floodplain approximately 6 km wide (p. 79), Aggradation Rates Not specifically mentioned, but aggrading within channel and on floodplain (p. 277) Degradation (p. 222) McCarthy et al. (1993) all sediment entering the fan is deposited and localized calcite precipitation leads to island development and aggradation. Nearly 300 meters in total. See above Not Mentioned 8-12 meters of sediment over last 300 ka or ~0.03 mm/y (p. 598), ~40 meters of sediment in total (p. 599) Likely very low as tectonically stable (p. 79), floodplain consists of up to 8 meters of sediment above pedogenically modified alluvium (p. 81) 26

39 System Narew River, Poland (Gradzinski et al., 2003) Baghmati River, India (Jain and Sinha, 2004) Klip River, eastern South Africa (Tooth et al., 2007) Magela Creek, Northern Territory, Australia (Tooth et al., 2008) Avulsion Mechanism Loss of channel capacity due to in-channel aggradation (p. 271) Loss of channel competence due to inchannel aggradation and tectonic adjustments (p ) Loss of channel capacity due to in-channel aggradation resulting from increased sinuosity (p. 456) Loss of channel capacity due to in-channel vegetation growth leading to deposition (p.1027) Splays and other Deposition No splays observed in area of study (p. 262), levees and alluvial ridges also not present (p. 273) Splays are observed and a high ratio of sediment to discharge (p. 166) Splays are not developed in study area (p. 455), small, discontinuous levees and alluvial ridges (p. 455) Splays are present, but no specific description given (p. 1028) Grain Sizes Mostly medium-coarse sand in channels (p. 263) and peat-rich floodplain (p. 264) Very-Fine to Fine sand in channels, intercalated with sandy silts and clays in overbank deposits (p in Sinha et al., 2005) Medium sand to fine gravel as bedload, and mud and fine sand as suspended load (p. 459) Fine and Medium sand, less than 15% fines (p. 1023, 1029) Floodplain Character Peaty floodplain (p. 264) that is laterally narrow, 1-4 km (p. 255) Seasonally wet/dry with floodplain inundated on average 35 days at depth of 1.48 meters (p. 351 in Jain and Sinha, 2005) Seasonally wet/dry wetland in region of moderate rainfall, but high evaporation (p. 454), less than 4 meters thick (p. 459) Savannah-like vegetation (p. 1024) and wet/dry monsoonal precipitation (p. 1023) Aggradation Rates Regional subsidence ~2 mm/y and net accumulation of peat is mm/y mostly over 1-3 ka (p. 255) mm/y of accumulation over ~1 ka (p. 166, after Sinha et al., 1996) Locally aggrading, but system is net degradational over 100 ka (p. 459) 1-2 mm/y for most of Holocene, but probably lower now (p ) 27

40 Figure 2.4: Stratigraphically Transitional and Abrupt Representations (after Jones, 2007). (A) Transitional stratigraphy features the coarsening-upward successions of heterolithic avulsion deposits (Kraus and Wells, 1999) consisting of finer predominately silt layers (greyish) and coarser predominately very fine and fine sand layers (tannish brown) overlain by channel deposits (tannish). Lateral splays similar to the coarse underlying splay sheets are also in abundance. This deposition style would feature extensive reservoir connectivity. (B) In abrupt stratigraphy, the channels are effectively incised directly into floodplain deposits (milky brown) due to the lack of heterolithic avulsion deposits, and lateral splays are either absent or rare with limited extent. Due to the predominance of floodplain mudstones, there is little reservoir connectivity. 28

41 3.0 Hypotheses and Study Phases 3.1 Hypotheses Hypothesis I Hypothesis I: Well-drained (dry and advective-transport dominated) floodplains will feature more extensive splay growth and deposition than poorly drained (wet and diffusive-transport dominated) floodplains. Adams et al. (2004) observed that the poorly drained and diffusive-transport dominated upper River floodplain featured narrower and steeper levees than the well-drained and advective-transport dominated Saskatchewan floodplain and traced these differences to the dominant mechanism of sediment transport. Given similarities in the development of levees and splays, this difference would be expected to extend to splay deposition, as well. In well-drained systems, steeper cross-floodplain water-surface slopes drive suspended sediments away from the channel margin and across the flood basin, resulting in large splay areas for a given channel discharge. In contrast, floodplains with flood-water levels commensurate with channel stage will not have the cross-floodplain water-surface slopes necessary to transport sediment far away from the channel margin. These systems will be associated with crevasse-splay deposits with relatively small planform areas for a given channel discharge Hypothesis II Hypothesis II: Systems featuring easily suspended and deposited intermediate grain-size distributions (generally coarse silt to fine sand for sand-bed rivers) will show more extensive splay development than systems with fine-grained (wash-load) or coarse-grained (bedload) dominated distributions. High discharge of splay-forming sediments through a crevasse should lead to larger crevassesplay areas for rivers of a given size. Intermediate grain-sizes are likely to both escape the channel flow and be deposited on the floodplain as flow expands away from levee crevasses. In contrast, while fine-grained sediments readily escape channel flow during floods, deposition of these sediments is a slow process requiring nearly stagnant flow, which is most common in distal 29

42 floodplain settings. On the other hand, coarse grain sizes typically occur as bedload and are not easily transported through crevasses onto the floodplain. 3.2 Study Components Three study components contributed to testing these hypotheses. The first involved Google Earth mapping of splay deposits in several modern systems including the Saskatchewan River system of Saskatchewan, Canada, a portion of the upper River system near Castledale, British, Canada, the Ovens River near Wangaratta, Victoria, Australia, and the Sandover- Bundey River system in the Northern Territory, Australia (Figure 3.1). In the second component, field observations of channel, splay, and floodplain deposits in the Fort Union Formation (Paleocene, Bighorn Basin, Wyoming), Willwood Formation (Paleocene - Eocene, Bighorn Basin, Wyoming), and Ferris Formation (Late Cretaceous - Paleocene, Hanna Basin, Wyoming) are compared (Figure 3.2). The third component focused on modeling varying grain-size distributions and two end-member floodplain-drainage conditions ( dry vs. wet ) using Delft3D-FLOW software from Deltares Systems. The following sections detail each of these studies. 30

43 Figure 3.1: Location Map for Modern Systems. (A) Locations of the upper River (C) and Saskatchewan River (S) reaches within Canada. (B) Locations of the Sandover River (S) and Ovens River (O) reaches within Australia. (Images are from Google Earth and copyrights are held by Google (A), TerraMetrics (A), Cnes/Spot Image (B), Whereis Sensis Party Limited (B), US Department of State Geographer (both), Data SIO (both), NOAA (both), US Navy (both), NGA (both), and GEBCO (both).) 31

44 Figure 3.2: Location Map for Wyoming Field Sites (after Jones and Hajek, 2007). The Paleocene Fort Union and Paleocene-Eocene Willwood Formations are in the Bighorn Basin, Wyoming, USA, while the Late Cretaceous-Paleocene Ferris Formation is in the Hanna Basin, Wyoming, USA. 32

45 4.0 Observations in Modern Rivers In order to evaluate how channel and floodplain conditions influence crevasse-splay development, I measured crevasse-splay dimensions and system characteristics in a range of modern avulsive systems using aerial photography within Google Earth. Specific measurements included length, width, shape, and orthogonal (to the channel margin) extent into the flood-basin for individual splays, and the length and area of channel reaches and floodplains. Collectively, these allow splay frequency and basin-filling properties to be characterized, which are important for determining how systems build levees and floodplains through overbank sedimentation, and, more broadly, may be key to understanding fluvial avulsions. To test my hypotheses, systems with varying grain-size distributions and floodplain drainage conditions were selected. 4.1 Study Areas Four fluvial systems with extensively studied reaches and varied channel, splay, and floodplain characteristics were identified from literature. These systems include the anastomosed reach of the upper River in British, Canada, the Cumberland Marshes region of the Saskatchewan River in Saskatchewan, Canada, a floodout zone of the Sandover River in Northern Territory, Australia, and the Deep Creek reach and vicinity of the Ovens River in Victoria, Australia (Figure 3.1). All of these systems have undergone recent avulsions, and so are useful for evaluating the role of crevasse-splay deposition in channel avulsion Upper River Makaske (1998) and Makaske et al. (2002) studied an anastomosed reach of the aggrading upper River near Castledale, British, Canada and mapped around 20 crevassesplay deposits. This reach s alluvial plain resides within an approximately kilometer wide depression between the slopes of the Rocky Mountains (northeast) and Purcell Mountains (southwest) and consists of numerous channels and lakes (Figures ). Given the laterally constricted floodplain and relatively wet environment, this system is poorly drained and standing water often accumulates across the floodplain during floods (Adams et al., 2004). Annual water discharge is around 108 m 3 /s (Water Survey of Canada, 1991, as referenced in Adams et al, 33

46 2004), but conditions are highly variable with high early summer flows due to snowmelt and rain (maximum discharge around June) and low winter discharges, when the channel contains ice (minimum discharge around February) (Makaske, 1998; Tabata and Hickin, 2003). The main channel mostly carries coarse and very coarse sand as bedload and medium sand as suspended load (Makaske, 1998; Abbado et al., 2005). Significant sediment extraction occurs within the anastomosed reach, as evidenced by bedload transport dropping from 7.1 kg/s to 0.6 kg/s in the 50 kilometer reach between Spillmacheen and Nicholson for measurements at near bankfull flood conditions (~220 m 3 /s) (Locking, 1983, as referenced in Makaske et al., 2009). This sediment extraction leads to in-channel aggradation of mm/year and levee aggradation of mm/year, with both rates decreasing downstream (Makaske et al., 2009). The measured bedload transport only accounts for 11% of total transport with the rest consisting of suspended and wash-load (Locking, 1983, as referenced in Abbado et al., 2005 and Makaske et al., 2009). Flood-basin aggradation estimates range from 1.7 mm/year (Makaske, 1998; Makaske et al., 2002) to 2.2 mm/year (Adams et al., 2004). The average downstream surface slope for the reach is approximately 15 cm/km (Adams et al., 2004). Makaske et al. (2002) estimated an avulsion frequency of approximately 3 per 1000 years, and, presently, there is an ongoing avulsion occurring within the downstream half of the anastomosed reach (Abbado et al., 2005) Saskatchewan River Studies by Smith et al. (1998) and Perez-Arlucea and Smith (1999) identified and characterized splays along the New and Centre Angling Channels of the Saskatchewan River (Figure 4.4). In this area (Cumberland Marshes), the floodplain for the entire system is up to 12 kilometers wide (Perez-Arlucea and Smith, 1999), and Adams et al. (2004) noted that water-surface elevations on the floodplain are below those of the channel, so they concluded this system is dominated by advective transport processes as seen on well-drained floodplains. Average annual water discharge is estimated at m 3 /s with annual maxima between m 3 /s occurring in July, following the spring thaw (Adams et al., 2004). Deposition within the Centre Angling reach was ongoing in 1945 and mostly complete by 1977 based on aerial photography (Perez-Arlucea and Smith, 1999). Grain sizes within channels and proximal splays are generally very fine to medium sand, but the overall most abundant grain size is medium to coarse silt 34

47 encountered in levees and most splays, with finer silt, clay, and peat formation observed in more distal and isolated settings (e.g. Smith et al., 1989). A Water Survey of Canada gauging station between Tobin Lake (Squaw Rapids Dam) and the study area recorded an annual suspended/wash-load transport rate of 2.9 kg/s, although this value is roughly 1% of the pre-dam (i.e. pre-1962) transport rate at Cumberland Marshes (Ashmore and Day, 1988). Smith et al. (1989) recorded a radiocarbon date of 2270 years before present for a peat layer beneath a 6.6 meter thick sequence of crevasse splay and levee deposits, which suggests an average aggradation rate of 2.9 mm/year. The average downstream surface slope within the Cumberland Marshes is approximately 10 cm/km (Adams et al., 2004). The Saskatchewan River is undergoing an avulsion that began in the early-mid 1870 s (e.g. Smith et al., 1989; Smith et al., 1998) and these New Channel (upstream of Steamboat Channel bifurcation) and Centre Angling Channel (downstream of the bifurcation) segments are emerging as the predominate thread following a heavily anastomosed phase (Smith and Perez-Arlucea, 2008) (Figure 4.4) Sandover River The Sandover River near Ammaroo, Northern Territory, Australia is a slowly aggrading, ephemeral, dryland system with documented splays (Tooth, 1999a; 2005) and avulsion (Tooth 1999a; 1999b; 2000a; 2000b; 2005; Tooth and Nanson, 1999; 2000). According to Tooth (1999a, p. 95), floodout is a common Australian term to designate an ephemeral stream reach where channelized flows end, but periodic floods still transport water downstream across the alluvial plain. In the floodout zone (the focus area for this and previous studies), the Sandover features a relatively wide (~6 kilometers), silty sand (30-40% silt-clay) floodplain and only periodic high discharge floods (every 3-4 years) (Tooth, 1999a) (Figure 4.5A). The active channel is generally 1-3 meters deep with steep banks, and transports medium to very coarse sand with gravel (Tooth, 1999a; 2005). Splays in the floodout zone are generally less than 0.5 km 2, and feature deeply erosive channels (up to 2 meters deep) in the proximal-medial reaches and limited interchannel deposition (Tooth, 2005). The distal ends of many splays consist of up to 0.4 meter high sediment lobes (Tooth, 2005). Similar depositional patterns are observed at the channel floodouts, although these features are generally larger in areal extent, and may contain finer sediments (Tooth, 1999a; 2005). Holocene alluvium is up to 6 meters thick in the study area 35

48 (Tooth, 1999a), suggesting aggradation rates of ~0.4 mm/year or less. Calculations using Google Earth yielded a downstream surface slope of (110 cm/km) for the studied reach, which falls in the range of for the arid Northern Plains provided by Tooth (1999a). In this area, there are two observed, recently active floodouts due to a 1974 avulsion that resulted in the infilling of the previously active channel reach with silty sand and very fine to medium sand (Tooth, 1999a) Ovens River Unlike the other studied rivers, no previous studies document the presence of splays on the avulsive Ovens River (e.g. Binnie and Partners, 1984; Rippin and Sheehan, 1986; Schumm et al., 1996; Cottingham et al., 2001; Judd et al., 2007), although the presence of alluvial ridges, natural levees, and channel margin crevasses is documented (Binnie and Partners, 1984; Schumm et al., 1996; Judd et al., 2007). While portions of the approximately 5 kilometer wide Ovens floodplain are anthropogenically modified through flood abatement strategies and agriculture (Binnie and Partners, 1984; Cottingham et al., 2001), an approximate 1.5 kilometer reach abandoned in 1974 (Judd et al., 2007) is evident in Google Earth, suggesting that the floodplain is not modified enough to obscure recent depositional and/or erosive features (Figure 4.5B). Channel discharge for the study reach is not specifically known, although stream gauges approximately 15 kilometers upstream (Rocky Point) and downstream (Wangaratta) have average instantaneous water discharges of 42.9 m 3 /s and 75.7 m 3 /s, respectively (Victorian Water Resources Data Warehouse, online data). This study examines the Pioneer, Deep Creek, and the Tarrawingee Reaches downstream of the Pioneer-Deep Creek bifurcation, because this area features a reach being abandoned (Pioneer), a reach developing through headward erosion (Deep Creek), and an active, mature reach (Tarrawingee) (Binnie and Partners, 1984; Schumm et al, 1996; Judd et al., 2007). Specific sediment load values are not known, although the river is classified as mixed load and floodwaters are relatively deprived of sediment (promoting headward incision of new channels) (Schumm et al., 1996). Vertical accretion is a slow process along the studied reach of the Ovens River and no estimates are provided (e.g. Judd et al., 2007). Downstream surface slope for the studied reach is approximately (180 cm/km) based on measurements within Google Earth, which is equal to the bedslope of the relatively straight and young Deep Creek 36

49 channel (Schumm et al., 1996). In this area, the channel beds consist of coarse sand and gravel and channel banks and floodplains consist of clay through gravel (Binnie and Partners, 1984; Schumm et al., 1996; Judd et al., 2007). Despite the wide apparent floodplain, the Pioneer Reach is flooded multiple times per year (Judd et al., 2007), suggesting poor drainage. 4.2 Google Earth Observations and Measurements For all rivers, reaches from published studies were located in Google Earth, and previously mapped splays and channel reaches were identified (predominately from: Makaske, 1998; Makaske et al., 2002; Smith et al., 1998; Perez-Arlucea and Smith, 1999; Tooth, 1999a; Tooth, 2005; Schumm et al., 1996). The character and surface presentation of previously identified splays (where present) was used a basis for splay identification in unmapped areas adjacent to mapped splays. The previously mapped splays and reaches are presented in Figures Measurements for each splay included length (along the primary crevasse-channel where apparent), width (generally orthogonal to crevasse-channel), parent channel width, orthogonal extent of the splay (perpendicular distance from the parent channel margin to the edge of the splay), and the latitude and longitude (WGS 84 coordinates) of the origin and terminus. Following mapping, each splay s polygon was saved in the.kml format provided within Google Earth. A representation of these measurements is presented as Figure 4.6. Once splays were mapped several other parameters were calculated to determine splay frequency and the percentage of the flood-basin filled by splays. These additional parameters include the length of the channel reach(es), and the area of channel(s), island(s), and flood-basins or floodout zones (i.e. the maximum extent of flooding for an ephemeral channel). Categorizing channel reaches required some interpretation on the classification of bifurcations around islands. This determination was made based on the daughter channel s width (w) and length (l). Where the w/l ratio is greater than 10, the daughter channel is considered a separate reach and is mapped and measured separately. Such determination was most necessary in the and Saskatchewan systems. Flood-basin boundaries (which also define study areas) are identified as the apparent orthogonal extent of floodwaters for the studied channel reaches, and this varies among systems. The Sandover is based on the limits of the floodout zone for the currently and recently active 37

50 channels of Tooth (2005). Defining flood-basins for channel reaches is harder in the other three systems because of the scale and complexity of the systems. Measurements in the Saskatchewan system focused on a partial reach of the New Channel/Centre Angling Channel (from the confluence with the Torch River to the confluence with the South Angling Channel) where the extent of the flood-basin was defined by the levees of the nearest adjacent primary channels (primarily the North Angling channel to the north and the South Angling or Gun Creek channels to the south). For the Ovens River, flood-basin extent was defined by the approximate floodplain limit to the north and the levee of the nearest abandoned channel (i.e. the method used for the Saskatchewan River) to the south. For the River, the floodbasin of the main channel (in the proximity of Castledale) was defined using the methods for the Saskatchewan and Ovens rivers. Since portions of the defined floodbasin are closer to adjacent, unstudied channels within these three systems, calculation of basin filling utilizing this definition might be seen as minimum estimate. With this in mind, an additional set of measurements was made for the upper utilizing the full width of the floodplain near Castledale, which is the area studied in detail by Makaske (1998) and Makaske et al (2002) (Figures 4.2 and 4.3). This floodplain width reach serves as a check on the values obtained using the method outlined for the Saskatchewan, Ovens, and s main channel reaches. Mapped lengths (i.e. splay channels, splay widths, parent channel widths, cross-sectional extent of splays, and channel reaches) and areas (i.e. splays, channels, islands, and flood-basins) were determined using the Calculate Polygon Area calculator at Further calculations to determine coverage percentages used simple arithmetic and statistical hypothesis testing to compare the studied fluvial systems utilized the Student t-test. 4.3 Google Earth Study Results General Appearance of Splays In the upper, larger splays show an elongate, finger-like appearance while smaller splays may show either an elongate form or a lobate form (Figure 4.7). In general, splays within the are widest and have the most channels near their origin and splays narrow and become single thread closer to their terminus. Splays that are poorly vegetated and appear sandy 38

51 are generally smaller with a more lobate appearance. Some splays show interconnectedness with adjoining splays, but most are isolated enough that wide flood-basin areas separate splays. Splays along the Centre Angling Channel of the Saskatchewan River follow the descriptions of Smith et al. (1989) (Figure 4.8). Smaller splays are normally isolated bodies with a lobate appearance and possibly anastomosed internal channel pattern, while larger splays show this internal anastomosed pattern and are connected to adjoining splays, including merging of primary crevasse-channels. Despite large-scale splay interconnectedness, individual splays are relatively easy to identify within the group. Unlike the system, the splay channels show a distributary pattern with a single or few channels near the origin, multiple bifurcations along the splay s length, and frequent merging of these distributaries to produce a downstream anastomosed pattern. Sandover River splays share some similarities with splays in the Saskatchewan system, but also some key differences. Splays are generally lobate, contain numerous, bifurcating channels, and are frequently connected to adjoining splays. However, single splays are hard to identify unless isolated (Figure 4.9). This is due to multiple bank breaches within short distances along channels. Thus, the more appropriate term for cataloged splays may be splay complexes, which consist of multiple small, highly interconnected splays in close proximity. For these reasons, a different categorization could lead to a vastly different splay count. The two documented floodouts of Tooth (1999a; 2005) are the features most similar to splays within the Saskatchewan system. These floodouts start as a few large distributaries that undergo many bifurcations and re-connections to show an anastomosed pattern downstream. Finally, portions of the floodout zone consist of pockets of banded reddish coloration similar to portions of active and recent splays (Figure 4.9), and these features might be representative of older weathered splay deposits. The Ovens River system contained no identified splays along any of the three studied reaches (developing Deep Creek anabranch, moderately mature Tarrawingee reach, and abandoning Pioneer reach) despite having an obvious pathway for the 1974 local avulsion and showing evidence for floodplain incisions. Figure 4.10 includes aerial documentation of these features. 39

52 4.3.2 Analysis of Splay Shape For each of the systems the number of identified splays, length of channel reach(es), and areal coverage of splays, channels, and other related features was determined and selected properties are provided in Table Splay Frequency and Basin Filling Of the three systems with identified splays, there are significant differences in both the system s propensity to develop crevasse splays and the basin-filling properties of these splays. Despite having a very small study area, the upper system creates the most splays per kilometer of channel length (2.10 km -1 ) and the Saskatchewan the least (0.878 km -1 ), as shown by the splay frequency statistic in Table 4.1. However, despite the upper s increased splay creation propensity, Saskatchewan splays fill much more of the floodbasin, covering 51.2% of the basin (as defined above). A middle value of 35.2% was obtained for the Sandover system, although this system s splays feature significant erosion and only limited deposition, primarily concentrated near the distal splay edge (Tooth, 2005). The upper despite a propensity to create many splays features splays that collectively only fill 17.6% of the studied basin. Additionally, similar values for splay frequency (1.82 km -1 ) and basin-filling (15.1%) in the upper were determined utilizing the entire floodplain width (i.e. Castledale Reach, as depicted in Figure 4.2), suggesting these results are not the product of sampling method. Collectively, these results corroborate qualitative observations made regarding the proximity and interconnectedness of splays in the preceding section, and may have implications for the predominate style of basin-filling within each system (splay development vs. flood-basin aggradation). Ultimately, the key results are that the Saskatchewan is producing expansive basinfilling splays, the Sandover is primarily producing erosive splays within the floodout zone, and the upper is producing many small splays that resemble nascent channels. Previous studies also examined the basin-filling characteristics of splay-related deposition in the Saskatchewan (Smith and Perez-Arlucea, 1994) and upper (Lavooi, 2010) systems. In the Saskatchewan, Smith and Perez-Arlucea (1994) estimated that large scale splay complexes comprised approximately 50-70% of the avulsion belt based on aerial photos and borings, which is in line with this study s result of 51%. Additionally, given this study s 40

53 conservative flood-basin definition and the New Channel s peripheral location within the avulsion belt, it s possible that splay basin-filling is under-represented in this study. For the upper system, Lavooi (2010) used borings along three profiles and aerial photography to determine facies proportions. She found that crevasse-splays made up approximately 4% - 20% of profiles, and that crevasse-splay deposition decreased downstream from Harrogate to Parson, which is also corroborated by Abbado et al. (2005). This study s estimate of approximately 18% is within the range of her estimates Splay Sizes and Shapes Measurements and calculations also documented individual crevasse-splay parameters including splay areas (Figure 4.11), parent channel widths (Figure 4.12), orthogonal extents into the floodbasin (Figure 4.13), maximum splay widths (Figure 4.14), and splay channel path lengths (Figure 4.15), with normalization of orthogonal extent by channel width (Figure 4.16) and determination of aspect ratio (path length divided by maximum width) (Figure 4.17). Summary statistics (mean, median, and standard deviations) for all of these parameters are presented in Table 4.2. Additionally, statistical hypothesis testing utilized Student t-tests (Table 4.2), and tested the null hypothesis that systems have the same parameter means. In each of the three systems, the relationships between splay area and parent channel width (Figures 4.18 and 4.19), orthogonal extent and parent channel width (Figure 4.20), and between splay channel path length and maximum splay width (Figure 4.21) were plotted. Collectively, these sets of measurements not only serve to document splay characteristics within the three fluvial systems, but also to provide a means of comparison among these systems. Review of Figures and Table 4.2 reveals the differences in crevasse-splay and system scale between the upper and the other two splay-producing systems. Splay areas in the are generally O[10 3 ] to O[10 4 ] meters 2 while they are primarily O[10 5 ] to O[10 6 ] meters 2 in the Sandover and Saskatchewan. Similar relative discrepancies in raw values generally hold for the other measured parameters, where high-end values in the are comparable to low-end values in the Sandover and Saskatchewan. Two additional observations from these plots and Table 4.2 include (1) the Sandover and Saskatchewan feature significant overlap in their raw value ranges, and (2) the plotted data are nearly log-normally distributed. 41

54 Using a t-test of the logarithm of splay area, it is calculated that there is greater than 95% confidence that mean splay area in the is different from that of either the Sandover or Saskatchewan. However, there is no statistically significant difference between the Sandover and Saskatchewan. Almost identical results are obtained when comparing the logarithms of parent channel widths. Thus, there is statistical evidence at the 95% level corroborating a difference in splay area and physical system scale for the when compared to the other two systems, but little evidence that there is statistical difference between the Sandover and Saskatchewan. Given these differences in system scale, it is appropriate to compare normalized values within the systems. Normalized orthogonal extent (by channel width) is presented in Figure 4.16, and it shows that most splays perpendicularly extend between 1 and 10 channel widths into their respective flood-basins in all systems a rather wide range of values. Taking the logarithm of normalized orthogonal extent and performing t-tests suggests there is no statistically significant difference between the three systems for this measure (Table 4.2). The spread of splay aspect ratios is a bit tighter than for normalized orthogonal extent, and most values are within the 1 to 6 range (Figure 4.17). T-tests performed on the logarithm of aspect ratio indicate and Sandover have statistically equivalent shapes (aspect ratios), but the difference between these two systems and the Saskatchewan is significant at the 95% confidence interval (Table 4.2). The Saskatchewan River is more likely to produce long and narrow crevasse-splays (i.e. higher aspect ratio). This result may be attributable to this system s tendency to produce compound splays featuring numerous smaller individual splays. Figures document relationships between measured parameters for individual splays. Figures show that there is virtually no correlation between splay area and channel width in any of the three systems measured. Likewise, orthogonal splay extent and channel width are unrelated in the and Sandover rivers, and shows only a weak positive relationship in the Saskatchewan River (Figures ). In summary, channel width is a relatively poor predictor of splay area and extent for the, Saskatchewan, and Sandover Rivers. 42

55 4.3.3 Splay Volume Estimates Using crevasse-splay thickness values presented in previous studies and the results presented in Sections and 4.3.2, it is possible to project the sediment volume deposited within splays for these three systems and more accurately evaluate basin-filling adjusted to system scale (i.e. discharge). Within the upper, Machusick (2000) documented the planform and sediment thicknesses of two modern splays, and other authors (e.g. Makaske, 1998; Makaske et al., 2002; Makaske et al., 2009; Lavooi, 2010) prepared basin-wide cross-sections that document and interpret crevasse-splay thicknesses for the modern and sub-surface. Machusick s maps indicate the tops of the Soles and Shalla Galla splays are mostly between and meters above the surrounding flood-basin, respectively, and eight cores within the Shalla Galla splay indicated thicknesses between 25 and 103 centimeters, suggesting similarity between measured surface levels and thicknesses. The Shalla Galla splay is within the Main Channel flood-basin studied in Section Trenches at Soles splay identified underlying flood-basin sediments within 65 centimeters of the surface. Mapped crevasse-splay thicknesses within other studies (Makaske, 1998; Makaske et al., 2002; 2009; Lavooi, 2010) are up to ~2.5 meters thick, but are mostly between 0.5 and 1.5 meters, with the thickest deposits possibly representing multi-storied deposits. Machusick (2000) and Makaske et al. (2002) reviewed aerial photos that indicate the splays developed on ~30 year timescales. Smith and Perez-Arlucea (1994), Perez-Arlucea and Smith (1999), and Farrell (2001) studied crevasse-splays of various sizes, ranging from Stage I Stage III development, within the Saskatchewan River system. The Stage III Windy Lake splay formed within a 35-year period and is mostly between 1.5 and 3 meters of thickness with an average thickness of 1.8 meters (Smith and Perez-Arlucea, 1994). The Stage III Cadotte complex is generally meters thick with an average thickness of 1.9 meters (Perez-Arlucea and Smith, 1999). This splay s development time is poorly constrained, but it is fully developed on the first aerial photos of the region taken approximately 65 years after the avulsion s start (Perez-Arlucea and Smith, 1999). Farrell (2001) mapped a small, Stage I splay off the Centre Angling Channel that is mostly 2 3 meters thick and fully developed within 25 years. This splay is within the studied reach analyzed in Section

56 Tooth (2005) studied splay development within several dryland systems in central Australia, including the Sandover system. These dryland splays typically feature splay channels incised up to 1.5 meters into the floodplain, with gradual decrease in incision depth towards the distal splay margins where net deposition is up to approximately 0.4 meters. In areas between these incised splay channels, net deposition ranges from meters with some splay channels being flanked by thin levees. Average deposition is not mentioned by Tooth (2005), but based on his descriptions and the low aggradation rate in this system (Section 4.1.3), it is likely that average splay thicknesses are no more than approximately 0.1 meters. Splay development may occur within single or few flood events, and the full timescale of splays evolution is likely a few decades as some splays observed in 1950 s aerial photography are no longer apparent (Tooth, 2005). Based on estimated average-splay thicknesses of 0.8 meters, 1.8 meters, and 0.1 meters, in the, Saskatchewan, and Sandover systems, respectively, the volume of crevassesplay sediment in the studied reaches (Section 4.3.1) is estimated and presented in Table 4.3. Additionally, a normalized splay volume ratio is presented for the and Saskatchewan. This ratio is calculated as (Average Splay Area [m 2 ] Average Thickness [m] / Average Development Time [years]) Sediment Discharge [m 3 /year] where the development time is estimated as 30 years for both systems. Estimated sediment discharges are m 3 /s and m 3 /s for the and Saskatchewan, respectively, based on estimated sediment fluxes of 65 kg/s (estimated from Locking, 1983, as referenced in Abbado et al., 2005 and Makaske et al., 2009) and 290 kg/s (estimated pre-dam construction from Ashmore and Day, 1988), respectively, and sediment bulk densities of 1,600 kg/m 3. These calculations show that the area of an average sized splay is two orders of magnitude less than in the Saskatchewan, and that this difference is not fully attributable to system scale, as normalized sedimentation in the is an order of magnitude less than in the Saskatchewan (Table 4.2). In other words, the relative diversion of River sediments into crevasse-splay deposits is an order of magnitude less than the relative diversion of river sediments into crevasse-splay deposits in the Saskatchewan system. [3] 44

57 4.4 Discussion Study of modern avulsive systems makes it apparent that there are significant differences in crevasse-splay shapes, scales, and basin-filling patterns even among similarly sized systems. The preceding results demonstrate that there are clear differences in splay propensity and basin coverage in avulsive fluvial systems. The upper system produces many small lobate and elongate finger-like splays, but the splays collectively cover less than 20% of the basin and feature extensive inter-splay areas of overbank deposition. On the other hand, despite producing fewer splays per given reach length, the Saskatchewan system features large, distributary-like splays that cover more than half of the flood-basin. Within the Sandover system, splay development is common and basin-coverage characteristics are intermediate between the and Saskatchewan, although splay channels themselves are highly erosive and splays generally feature only limited deposition at their margins and between splay channels. There is high interconnectivity between the erosive channels. Splays are not observed in the Ovens River system. The upper and Saskatchewan systems have similar net aggradation rates, longitudinal bedslopes, and have comparable sediment loads adjusted for system scale. Despite these similarities, splays within the Saskatchewan are much larger in planform and volume, and fill much more of the floodbasin than splays in the upper system. Adjusting for estimated sediment discharge in each system, the Saskatchewan splay volume is an order of magnitude larger than in upper splays, which means the discrepancy in areal size is not attributable to poor dispersal of overbank sediments leading to overly thick splays in the upper. Instead, the discrepancy between splay size (area and volume) in the Saskatchewan and upper may be attributable to processes that limit sediment supply from the channel to the floodplain. The upper features coarser grain sizes than the Saskatchewan (medium very coarse sand versus very fine medium sand), which may limit the amount of suspended material supplied to the floodplain through levee crevasses. Additionally, the upper features a poorly drained, laterally constricted floodplain, whereas, the Saskatchewan features a well-drained, wider floodplain (Adams et al., 2004). This difference would limit the potential for advective transport of overbank sediment within the, and may promote crevasse healing in the upper. Collectively, the differences in splay-deposit volume 45

58 relative to estimated sediment discharge between the upper and Saskatchewan systems implies that overbank sediment-supply to floodplains in the River is suppressed relative to the Saskatchewan. This may be the result of grain-size or floodplain-drainage differences between the two systems, or a combination of both. 46

59 Figure 4.1: Upper River Study Reach with Mapped Splays. Flow is from southeast to northwest (right to left). (A) The main channel s upstream reach is from near Harrogate to Castledale and features numerous splays, although splays are generally small and the reach features large intra-splay flood-basins. Splays mapped by Makaske (1998, his Figure 3.4) and repeated as Makaske et al. (2002, their Figure 4) are shown in yellow outline. Also note that this channel is not the predominate channel in the most upstream portion of the entire system reach (i.e. far southeast or far right in figure). (B) The downstream reach is similar to that of A, but fewer channels are observed in this downstream portion of the entire system reach. (Images are from Google Earth and copyrights are held by Province of British (A and B) and Image Parks Canada (A).) 47

60 Figure 4.2: Splays Mapped near Castledale. This figure identifies splays across the entire floodplain for the reach near Castledale, BC. This figure corresponds almost exactly with the area study of Makaske (1998, his Figure 3.4) and repeated in Makaske et al. (2002, their Figure 4), and documents splays identified within those studies. Notably, this study identified 43 splays within this reach (23.6 kilometers combined for identified channels), which is many more than those studies; however, this study identified several deposits as splays that were previously identified as levee extensions within those studies. In some cases, these bodies obviously grew between their identification and the date of this photo (circa 2005). Splay frequencies and splay basin-filling proportion are similar for this reach and the Main Channel Reach documented in the text, Table 4.1, and Figure 4.1. (Image is from Google Earth and copyrights are held by the Province of British.) 48

61 Figure 4.3: Comparison of Main Channel and Castledale Reaches. This figure shows the location of the Main Channel (Figure 4.1) and Castledale Reaches (Figure 4.2) as a means of comparison. (Image is from Google Earth and copyrights are held by the Province of British and Image Parks Canada.) 49

62 Figure 4.4: Saskatchewan River Study Reach with Mapped Splays. Flow is from southwest to northeast (left to right). (A) Smith et al. (1998, their Figure 4E) identified several splays (pink outline) along the New Channel of the Saskatchewan River, which is this study s upstream reach. This main thread continues as the Centre Angling Channel in the northern portion of A. Note the relative dearth of splays upstream of the Torch River confluence (far left), which is due to this area being on the periphery of the avulsion zone, and these splays are excluded from the reach analysis, although they are included in quantification of splay attributes (e.g. aspect ratio). (B) Perez-Arlucea and Smith (1994, their Figure 2B) identified several splays and associated abandoned channels along the Centre Angling Channel. Note the large whitish green areas between splays, which are not mapped with any specific splay, but may be associated with distal splay development (e.g. fen in Figure 2.3). (Images are from Google Earth and copyrights are held by Cnes/Spot Image (A and B) and DigitalGlobe (B).) 50

63 Figure 4.5: Sandover River and Ovens River Study Reaches with Mapped Splays. (A) Flow is from west to east in the Sandover system (left to right). The Sandover River s upstream floodout zone features numerous splays, most identified by Tooth (1999a, his Figures 9A and 13) and some also shown by Tooth (2005, his Figure 4A). Notable within this system is that major channels end in floodouts (purple outlines) at the downstream end of the study area, although unconfined flow continues downstream, as shown by the whitish tan coloration of the ground surface. (B) Flow is from southeast to northwest in the Ovens system (right to left). The three studied channel reaches Pioneer (yellow), Tarrawingee (orange), and Deep Creek (white) are outlined and numerous abandoned channel segments and reaches are evident on the floodplain (both within and outside the study area). Notably, no splay deposits are evident within this system. (Images are from Google Earth and copyrights are held by Cnes/Spot Image (A and B), DigitalGlobe (A), and GeoEye (B).) 51

64 Figure 4.6: Splay Measurements Made within Google Earth. Numerous measurements were made within Google Earth. These included measuring the splay length, which was typically done by measuring along the primary splay channel s path, where evident. Other measurements included the coordinates (WGS 84) for the splay s origin and terminus, splay width, parent channel width at the splay site, and the orthogonal extent. The orthogonal extent measures the width of the splay s extent into the flood-basin perpendicular to the parent channel. (Image is from Google Earth and copyright is held by Province of British.) 52

65 Figure 4.7. Splay Examples from the Upper River. (A) Small lobate splays are approximately 100 x 100 meters or less and feature a handful of small crevasse channels. (B) Elongate splays feature a fan-like origin and finger-like channel extension. Length scales are generally several hundred meters for these splays. Within both types, a portion of the mapped splays are likely below water level during floods. Machusick (2000) linked this dimension and planform variability to whether flood-basins contained downstream outlets that acted to guide flow, leading to elongate planforms. (Images are from Google Earth and copyrights are held by Province of British.) 53

66 Figure 4.8: Splay Examples from the Saskatchewan River. Splays within the Saskatchewan system generally feature a predominate channel emerging from a crevasse with multiple bifurcations downstream leading to splay anastomosis. Downstream extensions frequently feature a merging of distributary channels from initially distinct splays (e.g. right-center of photo). Splay development in this system resembles what might be expected in the Sandover with more cohesive floodplain sediments. Note how large splays develop long and narrow planforms, which contributes to the aspect ratio difference of the Saskatchewan with respect to the other systems. (Image is from Google Earth and copyrights are held by Cnes/Spot Image.) 54

67 Figure 4.9: Splay Examples from the Sandover River. (A) Splays within the Sandover system result from numerous, closely-spaced channel bank breaches. There is a high degree of inter-connectedness between these channels. Splay development in this system resembles what might be expected in the Saskatchewan with less cohesive floodplain sediments. Note the presence of the abandoned channel (pre 1974 avulsion) in the lower right (southeast) portion of the photo. (B) Proximal floodouts are similar to splays in deposition and growth, and feature main channels with many bifurcations leading to small distributary channels. Note that the floodout defined here is the depositional (splay-like) portion and that small channel pathways are evident downstream of the defined boundary. These small channels continue for approximately 15 kilometers before coalescing into another primary channel. (Images are from Google Earth and copyrights are held by DigitalGlobe (A and B).) 55

68 Figure 4.10: Channel Development and Floodplain Incisions along the Ovens River. (A) The Ovens River does not feature splay development, although the channel pathway prior to a 1974 localized avulsion is apparent (yellow arrows). Additionally, the headward (upstream) channel development of the Deep Creek reach is evident, as the downstream portion is much wider (aqua arrow) than the upstream portion (blue arrow). (B) Thin floodplain incisions or gullies are evident on this portion of the Ovens River floodplain. These features may serve as the development locations for new anabranches or portions thereof. The red arrows mark the same location on A and B; thus, the continued upstream development of Deep Creek can be observed. (Images are from Google Earth and copyrights are held by GeoEye (A and B).) 56

69 Table 4.1: Measured Splay Quantities in Modern Systems. The following are the results of the Google Earth analysis. Splay frequency represents the number of splays per kilometer of reach length. Splay proportion is the amount of the basin filled by splays when excluding channel areas. System Studied Reach Length (km) Identified Splays Splay Frequency (km -1 ) Study Area Size (km 2 ) Channel Area (excludes Islands) (km 2 ) Splay Area (km 2 ) Channel Proportion Splay Proportion (excluding channels) River* * Sandover River ** Saskatchewan River *** Ovens River * -Values are for Main Channel Reach only. 33 additional splays were measured near Castledale (Figure 4.2). **-Includes two floodouts. ***-Values are for described reach only. 4 additional splays were measured along the New Channel before the confluence with the Torch River. 57

70 Figure 4.11: Splay Areas within the Three Systems. Splays areas for identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 58

71 Figure 4.12: Channel Widths within the Three Systems. Parent channel widths for identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 59

72 Figure 4.13: Orthogonal Extent into Flood-basin within the Three Systems. The orthogonal (with respect to the parent channel) extent splays extend into the flood-basin for identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 60

73 Figure 4.14: Measured Splay Widths within the Three Systems. Maximum splay widths for identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 61

74 Figure 4.15: Splay Channel Path Lengths within the Three Systems. Lengths of the primary splay channel for identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 62

75 Figure 4.16: Normalized Orthogonal Extent (by Channel Width). Orthogonal extent of identified splays adjusted for channel scale (width) in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 63

76 Figure 4.17: Aspect Ratio (Maximum Splay Width/Splay Channel Path Length). Aspect ratio (length/width) of identified splays in the upper (n = 76), Sandover (n = 32), and Saskatchewan (n = 43) River systems. 64

77 Table 4.2: Summary Statistics and T-test Comparison Results. Means, Medians, and Standard Deviations are presented for each physical parameter measured in identified and mapped splays. Also presented are the results of two-tailed, heteroscedastic (i.e. assumes unequal variances for the different systems) Student t-tests that compare each of the systems. For the t-test, the null hypothesis is that the means of the parameters are equivalent in the corresponding systems. System or Comparison (n = 76) Sandover (n = 32) Saskatchewan (n = 43) vs. Sandover vs. Saskatchewan Sandover vs. Saskatchewan Summary Statistic Splay Area (m 2 ) Log of Splay Area Channel Width (meters) Log of Channel Width Orthogonal Extent (meters) Maximum Splay Width (meters) Splay Channel Path Length (meters) Normalized Orthogonal Extent (by Channel Width) Log of Normalized Orthogonal Extent Aspect Ratio (Splay Length divided by Maximum Width) Log of Aspect Ratio Mean Median Standard Deviation Mean Median Standard Deviation Mean Median Standard Deviation Student t- test Probability Student t- test Probability Student t- test Probability Non- Normal Non- Non- Non- Non- Non Non-Normal Normal Normal Normal Normal Normal

78 Figure 4.18: Splay Area vs. Channel Width in the Upper System. Scatter plot of the raw splay area and channel width values in the upper. 66

79 Figure 4.19: Splay Area vs.channel Width in the Sanodver and Saskatchewan Systems. Scatter plot of the raw splay area and channel width values in the Sandover and Saskatchewan. 67

80 Figure 4.20: Orthogonal Extent vs. Channel Width in the Three Systems. Scatter plot of the raw splay orthogonal extent and channel width values. 68

81 Figure 4.21: Splay Channel Path Length vs. Maximum Splay Width in the Three Systems. Scatter plot of the raw splay lengths and widths. 69

82 Table 4.3. Estimated Splay Volumes and Normalized Splay Discharge. Using average splay areas determined in this study (Section 4.3.1) and estimated splay thicknesses from literature (Section 4.3.3), the sediment volume deposited in the studied reaches and an average sized splay is calculated. Further, a normalized sediment discharge is estimated and presented to account for splay development time (~30 years), and estimated sediment discharges in these systems (0.041 m 3 /s and m 3 /s, respectively). Since the Sandover is not gauged and discharge is unknown, this system is excluded from the normalized calculations. "Normalized Sediment Discharge" is an estimate of the fraction of total sediment discharged diverted into crevasse-splay deposits in the and Saskatchewan rivers. System Total Splay Area (m 2 ) Average Splay Thickness (m) Estimated Total Splay Volume (m 3 ) Average Splay Area (m 2 ) Estimated Average Splay Volume (m 3 ) Normalized Sediment Discharge 1,060, ,000 19,500 16, x 10-4 Sandover 35,100, ,500,000 1,110, , Saskatchewan 33,400, ,000, ,000 1,700, x

83 5.0 Observations in Ancient Systems Ancient deposits provide an important opportunity to study the relationships between grain-size, floodplain drainage, and splay deposits. Thus, during the summer of 2011, five weeks of field study focused on determining how grain-size distributions and floodplain drainage is related to crevasse-splay size and abundance in several ancient fluvial systems. In particular, this study examined the Late Cretaceous-Paleogene Ferris Formation (Hanna Basin, south-central Wyoming), Paleocene Fort Union Formation, and Paleocene-Eocene Willwood Formation (Bighorn Basin, north-central Wyoming) (Figure 3.2). Jones and Hajek (2007) described the Ferris Formation as stratigraphically abrupt with limited splays and the Fort Union and Willwood formations as stratigraphically transitional with abundant splays. 5.1 Formations Ferris Formation The Late Cretaceous-Paleocene Ferris Formation was deposited during the Laramide orogeny within the intermontane Hanna Basin (e.g. Jones, 2007; Jones and Hajek, 2007; Hajek et al., 2010b). Fossil evidence suggests the climate was tropical-temperate and the setting was deltaic lacustrine to estuarine (Wroblewski, 2004). Eberle and Lillegraven (1998) examined approximately 660 meters of Ferris section fully within the Puercan North American land mammal age (circa 65 Ma Ma), suggesting an accumulation rate of ~0.4 mm/year or more (Hajek et al., 2012). Within the formation, channel-belt sand bodies are generally tens to hundreds of meters wide, meters thick, and contain 1-2 stories (Jones, 2007). Sand body grain sizes are coarse to very coarse sand and the average interpreted paleoflow depth is 0.6 meters (Hajek et al., 2012). Crevasse-splays are rare throughout the Ferris Formation and are never observed in the interval below a channel. Instead, channel deposits cut into and directly overlie dark carbonaceous floodplain mudstones (Jones, 2007; Jones and Hajek, 2007). 71

84 5.1.2 Fort Union Formation The Paleocene Fort Union Formation also dates to the Laramide orogeny and was deposited within the intermontane Bighorn Basin (e.g. Bown, 1980; Kraus, 1998; Kraus and Wells, 1999). The depositional climate was humid continental with a mean annual temperature of approximately 10 C and a temperature range of 25ºC (Hickey, 1980). The formation is predominately fluvial, but areas of paludal and lacustrine conditions are known (Hickey, 1980). Kraus and Wells (1999) estimated basin-averaged sedimentation rates to be ~0.1 mm/yr during early Fort Union deposition and increasing to ~0.3 mm/yr by later stages of deposition. Larger trunk channel deposits are normally around 10 meters thick with 2 to 3 stories that are each 3 to 4.5 meters thick, suggesting paleoflow depths of 3-5 meters (Kraus, 1998; Kraus and Wells, 1999). These trunk channels are typically around 1 km wide with predominate grain sizes from very fine to (rarely) coarse sand, and are interpreted as meandering channels (Bown, 1980; Kraus, 1998; Kraus and Wells, 1999). Smaller distributary and crevasse-channel deposits are relatively narrow, generally single or weakly multi-story ribbon sandstones on the scale of tens of meters wide, typically 1 to 3 (up to 9) meters thick with grain-sizes from very fine to medium sand (generally finer) (Kraus and Wells, 1999). Trunk channels are commonly underlain by heterolithic avulsion deposits (Kraus and Wells, 1999) comprising abundant thin very-fine grained sandstones less than 1 meter thick encased in weakly pedogenically modified silty deposits found both lateral to and beneath paleochannels are collectively interpreted to represent crevasse-splays and related overbank deposition (Kraus, 1998; Kraus and Wells, 1999). Clay and silt distal floodplain deposits not associated with avulsion sequences feature stronger pedogenic development with yellow, brown, and black soils, which suggests slower aggradation rates (Kraus, 1998) Willwood Formation The Paleocene-Eocene Willwood Formation conformably overlies the Fort Union in the Bighorn Basin (although some peripheral basin areas feature an angular unconformity; Bown, 1980). During deposition climate shifted towards warmer and dryer conditions with mean annual temperatures around 13.5 C to 17 C and winter temperatures above freezing during the Eocene (Hickey, 1980; Greenwood and Wing, 1995; Kraus and Wells, 1999). Basin-averaged 72

85 sedimentation rates within the Willwood Formation are estimated at ~0.5 mm/yr (Kraus and Wells, 1999). Sedimentology within the Willwood Formation is similar to that of the Fort Union Formation (Kraus and Wells, 1999), with similar facies characteristics for channel bodies, avulsion-related deposits, splays, and other proximal overbank deposits (e.g. Kraus and Aslan, 1993, Kraus, 1996, Kraus, 1998; Kraus and Wells, 1999). The primary sedimentological difference between the two Bighorn Basin formations is the degree of paleosol development in the Willwood Formation (Kraus, 1998; Kraus and Wells, 1999). The Willwood Formation is characterized by its distinctive, well-developed paleosol horizons and successions (e.g., Kraus, 1996). These several meter-scale successions feature a basal light grey or greenish-gray siltstone overlain by a finer-grained (i.e. higher proportion of clay) yellow-brown paleosol horizon. The succession is capped by a clay-rich heavily mottled bright red and grey clayey paleosol horizon (Kraus, 1996). This succession reflects hiatal intervals during which thick, well-developed soils formed on the floodplain and likely captures an overall decline in sedimentation rates throughout the sequence (e.g. Bown and Kraus, 1987). 5.2 Field Observations In order to test my hypotheses, field observations focused on obtaining information about paleosediment loads and paleo-floodplain drainage conditions, as well as crevasse-splay deposit characteristics in each formation. Visual grain-size estimates and hand samples were collected from channels, proximal overbank deposits (including splays and avulsion deposits), and distal overbank (i.e. floodplain) deposits. The thickness and lateral extent of these deposits was also measured and/or estimated, and paleosol development was described within overbank deposits. Collectively, these measurements test the stated hypotheses in three primary ways: (1) determination of the range of grain-sizes present, (2) the relative proportions of specific grainsizes in the deposits, and (3) relative floodplain sedimentation rates and saturation conditions. Sampling at each outcrop included collection of three or more hand samples from paleochannels to document grain sizes at the base, middle, and top of the deposit and any notable structures within the deposit (e.g. mud-plugs, sand bars, etc.). Multiple samples collected from individual interbeds within avulsion deposits preceding channels. Sampling of individual horizons including splay sand-sheets and paleosol successions is in the form of vertical 73

86 sections with increasing distance from the channel. A representation of this sampling pattern in presented as Figure 5.1. In the late summer and autumn of 2011, analytical grain-size distributions were obtained for characteristic channel, splay, and floodplain samples. Sample preparation consisted of weighing, disaggregation using a mortar and pestle, and dry sieving. Care was taken to ensure that as few grains as possible would be crushed or cracked during disaggregation and sieving, although this is a potential source of error. Dry sieving separated the disaggregated samples into fractions finer and coarser than 850 microns (0.85 mm), equivalent to upper coarse sand. Particle-size measurements of fine disaggregated samples were conducted at Penn State's Particle Characterization Laboratory (PCL) using the Malvern Mastersizer S. The Mastersizer S uses dynamic light scattering with a Helium-Neon laser (633 nm wavelength) to determine particle sizes ranging from 0.05 to 900 microns (0.9 mm). Prior to testing, each sample vile was filled with approximately 50 ml of water, placed in an ultrasonic bath for approximately 2 minutes to further disaggregate fine sediment, and hand shaken to suspend particles. With the finer sediment suspended, a portion of the sample was introduced to the Mastersizer via the wet sampler. A Malvern program then calculates and reports the percent volume of particles in each of 64 grain size intervals between 0.05 to 900 microns. These values were saved in a spreadsheet for further analysis. 5.3 Results Ferris Formation Channel Facies: Two sampled single story channel deposits were ~ 0.6 and 1.8 meters thick vertically and extend on the order of several tens of meters laterally. Visually estimated Grain sizes within channel bodies range from upper medium sand to upper very coarse sand (~ microns) with most in the upper coarse to lower very coarse range (~ microns) (Figures 5.2 and 5.3). Proximal Overbank Facies: Channel bases either directly overlie dark carbonaceous floodplain shales or thin (10 20 centimeters thick) interbedded clay-silt-sand horizons with most sediment in the silt to lower very fine range (~4 125 microns) (Figures 5.2 and 5.3). Notably, when channels overlie these thin vertical successions the contact is mostly non-erosive and 74

87 conformable, and the underlying vertical succession takes the same shape as that of the channel base, suggesting these deposits may represent the nascent development of the channel. At the lateral margins of channels, interbedded sand and clay silt intervals (~1 30 centimeters thick) rapidly thin and fine away from the channel, pinching out within meters of the channel margin. A lower fine to upper coarse (~ microns) splay sand up to 30 cm thick pinched out within only 7 meters. These deposits may reflect restricted channelmargin and crevasse-splay deposition. Distal Overbank (Floodplain) Facies: The formation s most common facies is dark carbonaceous, peaty and coaly shale with little paleosol development. This is characteristic of swampy and waterlogged conditions (e.g. Mack et al., 1993). Distal overbank grain sizes are mostly in the clay to fine silt range (~1 16 microns), with small amounts (<25%) of medium silt to very fine sand ( microns) (Figure 5.3) Fort Union Formation Channel Facies: Observations included two trunk channels with grain-sizes ranging from very fine sand to pebbles (Figure 5.2). Pebbles (~2 64 mm) were only found in numerous sand bars (roughly meter thick) in a single trunk channel with other non-bar portions in the fine to medium sand (~ microns) range. The other observed trunk channel contained very fine to medium sand (~ microns) (Figure 5.3). The trunk channels were several hundred meters to a kilometer or more in width and 6 to at least 10 meters thick. Trough cross-bed sets within the channel bodies were centimeters thick (truncated). Proximal Overbank Facies: Ubiquitous coarsening-upward successions of interbedded silt and sand layers are found beneath paleochannels, which are the heterolithic avulsion deposits of Kraus and Wells (1999). Grain sizes range from clay to fine sand (~1 250 microns) in these intervals (Figure 5.4), and they feature weak paleosol development (rare orange mottling), which suggests rapid deposition (e.g. Kraus, 1998). Widths and thicknesses of proximal overbank facies sequences are on scale with the overlying channel deposits. A splay channel within a heterolithic complex was also measured and characterized. It was between 2 and 4 meters thick and around 30 meters wide. Grain-sizes within the channel 75

88 ranged from fine to coarse sand (~ microns). Notably, this ribbon channel featured a full succession of lateral paleosols from weakly to heavy modified. Lateral to channel facies, proximal overbank deposits include splay sandstone sheets and fine-grained intervals. Sandstone sheets are very fine to medium sand (~ microns) in decimeter scale beds (up to near a meter) with ripple laminations. These sheets extend for tens to hundreds of meters, although the full extent cannot be measured due to exposure. Finer-grained intervals generally consist of clay and silt and feature weak or moderate paleosol development (greenish grays to orange), suggesting fairly rapid deposition, although slower than for heterolithic deposits. Lateral extent is on the order of hundreds to thousands of meters. Distal Overbank (Floodplain) Facies: Distal floodplain sediments had varying paleosol development with one outcrop consisting of muted red, mottled paleosols and another outcrop consisting of blackish, brownish, and yellowish paleosols. These collectively suggest relatively low deposition rates (when compared with proximal overbank horizons) and some seasonality in exposure, but the latter type is indicative of more waterlogged conditions and is the more common type observed in the formation by others (e.g. Bown, 1980; Kraus, 1998). Grain sizes within distal floodplain deposits are clay to fine silt. Individual horizons are meter-scale in thickness and extend laterally for hundreds to thousands of meters Willwood Formation Channel Facies: The five Willwood Formation channel deposits feature grain sizes from very fine to very coarse sand (~ microns) (Figure 5.5), although most samples were in the fine to medium sand range (~ microns). A 0.9 meter sand bar was observed, and contained the coarsest grains observed in the Willwood Formation. Trough cross-beds up to approximately 50 centimeters in thickness were measured. Overall, channels were between 2.8 and 6 meters thick, and lateral extents were up to a kilometer and possibly more, although exposures were not complete. All identified channels were trunk channels (e.g. Kraus and Wells, 1999). Proximal Overbank Facies: The Willwood Formation features at least 2.5 meter thick sequences of coarsening-upward heterolithic avulsion deposits consisting of interbedded silt and sand (Kraus and Wells, 1999). Grain sizes range from clay to fine sand (Figure 5.5), and very 76

89 weak paleosol development (rare orange mottling), suggesting rapid deposition (e.g. Kraus and Aslan, 1993; Kraus, 1996). These features frequently extended below the ground-surface. Proximal overbank deposits lateral to channel deposits include splay sandstone sheets and fine-grained intervals. Observed sand sheets are slightly finer in the Willwood Formation compared to the Fort Union Formation, and feature very fine to fine sand in ripple-laminated, centimeter thick beds. These sandstone sheets extend tens to hundreds of meters away from channels. Finer-grained intervals had clay and silt grain sizes, weak to moderate paleosol development (greenish grays to orange), and lateral extent on the order of hundreds to thousands of meters. Distal Overbank (Floodplain) Facies: These facies feature the most significant differences between the Fort Union and Willwood formations. Grain sizes in Willwood floodplain deposits are entirely clay to fine silt (~1 16 microns) (Figure 5.5), and distal overbank paleosols are always reddish and purplish with extensive mottling featuring various colors (greens, oranges, purples, and reds) and slickenslides. This development suggests extensive seasonal wetting and drying (e.g. Mack et al., 1993). Lateral extent of these deposits is from hundreds to thousands of meters, and individual horizons are between approximately 0.5 and 2 meters thick. 5.4 Discussion Crevasse-splay development differs between the Ferris Formation and the Fort Union and Willwood formations as evidenced by the frequency of observed splays and their lateral extent. The Ferris Formation features few and laterally limited splays with none observed beneath channels, while splays are common, extensive laterally, and ubiquitous beneath channels in the Fort Union and Willwood Formations. These differences are potentially due to discharge, available grain sizes, and floodplain drainage characteristics of the formations. As presented in the preceding section, the channel scale within the Ferris Formation is smaller than that of the Fort Union and Willwood Formations, which may contribute to differences in the scale of splays. Individual Fort Union and Willwood channels are approximately 2 5 times deeper than Ferris channels and several times wider, which may account for some of the discrepancy in splay sizes. However, deposits lateral to Ferris channels pinch out in a distance equivalent to or less than the system s channel width, while horizons 77

90 within the Fort Union and Willwood Formations are traceable for distances well in excess of the channel width (e.g. Kraus and Wells, 1999). This suggests that other factors play a role in the lateral extent of deposits. Overall, the Ferris Formation comprises an approximately bimodal grain-size distribution with peaks in coarse sand and clay finer silt, and very little intermediate sized sediments. Laterally narrow (several tens of meters) and thin paleochannel bodies consist predominately of coarse and very coarse sand, while most overbank deposits clay and fine silt floodplain intervals. Presumably, coarse in-channel deposits reflect characteristic bedload in the Ferris Formation, while the clays and fine silts on the floodplain reflect the paleo-washload. Rare crevasse-splay or channel-margin deposits range from coarse silt to fine sand, and may reflect a relatively small fraction of coarse suspended load in Ferris rivers. In contrast, the Fort Union and Willwood Formations generally consist of a narrower, more unimodal distribution with a range of grain sizes predominately from clay to medium sand (with minor coarse sand to pebbles). Approximately kilometer-wide channels consist primarily of fine and medium sand, which, for rivers with few meter scale flow depths, would likely have been easily suspended (e.g. Shah-Fairbank et al., 2011). Crevasse-splay deposition in these formations is abundant, both beneath channels in the form of the ubiquitous "heterolithic avulsion deposits" of Kraus and Wells (1999) and lateral to channel deposits as overbank deposits. Floodplain-drainage conditions in the Ferris Formation also differ markedly from those in the Fort Union and Willwood Formations. Floodplains in the Ferris Formation show little paleosol development and feature highly carbonaceous and peaty, fissile shale, which suggests waterlogged conditions and rapid flood-basin sedimentation (Mack et al., 1993). On the other hand, distinct paleosol development in both the Fort Union and Willwood Formations consists of deep red paleosols with heavily mottled gray, green, and orange coloration, suggesting seasonal wetting and drying, and periods of low sedimentation (e.g. Simonson and Boersma, 1972; Mack et al., 1993). The Fort Union Formation also features some outcrops displaying black and yellowish paleosols, indicating wetter and more acidic conditions (Kraus, 1998). Differing floodplain inundation, sedimentation rates, and climate may account for these paleosol differences. 78

91 In general, observations in these formations validate the hypotheses that large and abundant splays will form most readily in systems that (1) have dry (at least seasonally) floodplains (facilitating a lateral water-surface slope across the floodplain) and (2) abundant coarse suspended sediment (e.g. coarse silt fine sand) that is delivered to the floodplain via levee breaches. These conditions prevail in the Fort Union and Willwood Formations, which feature large splays, periodically dry floodplains, and an abundance coarse silt to fine sand, while the Ferris Formation, with small, rare splays, had persistently wet floodplains and very little coarse silt fine sand. These results are consistent with observations from modern systems (Section 4.0). 79

92 Figure 5.1: Representation of Field Sampling. In the field, samples were collected from paleo-channel bodies, avulsion deposits beneath channels, and in multiple vertical sections with distance away from the channel. Within these sections, samples were collected from the various paleosol horizons and lateral avulsion deposits. 80

93 Figure 5.2: Range of Visually Estimated Grain-Sizes for Facies Types. The marker point represents the approximate visual mean grain-size estimate for an outcrop and the vertical bars represent the visual grain-size range for a specific outcrop. 81

94 Figure 5.3: Example Ferris Grain-Size Distribution. Due to limitations in the Mastersizer, the channel sample was tested using the CAMSIZER at Tulane University. 82

95 Figure 5.4: Example Fort Union Grain-Size Distribution. 83

96 Figure 5.5: Example Willwood Grain-Size Distribution. 84

97 6.0 Modeling Splay Development in Delft3D Observations in modern and ancient systems presented in the preceding sections are consistent with the hypotheses that greater splay development occurs in systems featuring intermediate grain-size distributions and dry, well-drained (i.e. steep water-surface gradient) floodplains. However, these cannot help determine the importance of either grain-size or floodplain drainage on splay production in isolation, or alternatively, what specific combinations of grain-size and floodplain drainage will produce the most abundant and largest splays. In order to further evaluate the importance of these variables, I used Delft3D to: (1) document differences in splay morphology on dry versus wet floodplains (i.e. those dominated by advective or diffusive sediment transport) and (2) document differences resulting from three different grain-size distributions including a fine distribution (primarily clay and very fine to fine silt), an intermediate distribution (primarily coarse silt and very fine to fine sand), and a coarse distribution (primarily medium and coarse sand). Modeling follows the abbreviated workflow provided in Figure 6.1. There are two modeling domains to account for advective and diffusive flow routing from which each grain size distribution is modeled for a total of six final models. 6.1 Methods This modeling was conducted used the software package Delft3D-FLOW from Deltares Systems. Delft3D is fully validated for modeling the 3-dimensional depth-averaged, non-linear, shallow water Navier-Stokes equations in accordance with the computational standards of the International Association of Hydraulic Research (IAHR, 1994). Six model runs with varying sediment distributions and floodplain drainage (Figure 6.1) were conducted on the same basic grid (Figure 6.2). The model setup assumes the channel is in near-equilibrium and graded conditions in the absence of the crevasse, as shown by model results discussed below in Section Equations 4 9 below are solely utilized in determining parameter inputs in the Delft3D- FLOW interface, as fluid and sediment transport conditions are calculated within the Delft3D- FLOW program itself. 85

98 6.1.1 Model Grid and Initial Topography The grid and bathymetry were constructed using Delft3D-RFGRID and Delft3D-QUICKIN, respectively. Important parameters for the grid and bathymetry are provided in Table 6.1 and the modeled grid and bathymetry are provided in Figure 6.2. To reduce the model computational time but still allow for accurate morphodynamic and hydrodynamic estimates the model incorporates a variable grid size and features a wider grid in the center of the domain to simulate the floodplain. Upstream and downstream of this wider grid, the grid is only 75 meters wide (15 grid cells in y-direction) to simulate the channel and levees. Within the simulated floodplain area, cell dimensions are 5 x 5 meters in a 1,000 x 1,000 meters area in vicinity of the crevasse and increase outward away from this area. In total, the wide central grid measures 3,067 (downstream) x 2,634 meters (orthogonal to downstream direction) and is 267 x 251 grid cells. Within the channel, the grid cells are always 5 meters wide orthogonal to the flow direction, but are variable longitudinally with a total channel length of 23,550 meters (321 grid cells, including the 267 cells discussed above in the middle reach). The modeled bankfull channel is 60 meters wide, has a bankfull depth of 3.0 meters, a floodplain elevation that is halfway between the channel bottom and levee height, side slopes in a 1:1 ratio, and transports a flow in the positive x-direction. The downstream channel bedslope is initially m/m. The left levee is two grid cells wide (10 meters) and the right levee is one grid cell wide (5 meters) and lies on the edge of the grid domain. Each levee cell is assigned a height 3.0 meters above the base of the channel. The crevasse is cut through the left levee and is located approximately 11,270 meters downstream of the upstream boundary, is ~10 meters wide (1 full/2 partial grid cells in the downstream x-direction), ~15 meters long (2 full/2 partial grid cells in the cross-channel y- direction), and the base is at floodplain height (i.e. ~1.5 meters above the channel bed). For ease of calculations, this floodplain does not feature a bedslope. Additionally, a second bathymetry featuring no crevasse was prepared for use in preliminary hydrodynamic and morphological testing Initial Bed Sediments In Delft3D, sediment classes are user-defined and are either cohesive or non-cohesive, with cohesive sediments being silt-sized and finer (< 64 microns). Cohesive sediment erosion and 86

99 deposition is calculated using the Partheniades-Krone formulations based on user-defined critical shear stress thresholds, while cohesive sediment transport is determined via the advectiondiffusion equation. Cohesive sediment classes are determined based on settling velocity, which for this study used Stokes Law ω = 2 9 (ρ p ρ f ) μ gr 2 [4] where ω is particle settling velocity, ρ p is the particle density, ρ f is the fluid density, μ is the fluid dynamic viscosity, g is gravity, and r is the particle radius. For quartz particles (ρ p = 2650 kg/m 3 ) in 20 C water (μ = 10-3 N s/m 2 ), the settling velocity is approximately ω = D 2 [m/s] [5] or ω = 900D 2 [mm/s] [6] where D is the particle diameter. Within this model, there are three classifications for cohesive sediments, including categories for clay, finer silt (very fine and fine silt), and coarser silt (medium and coarse silt) (Table 6.2), for which the critical shear stress for entrainment is set. The listed settling velocities correspond to grain diameters of 1.95, 7.81, and microns for these classifications, respectively. Hydrodynamic testing in a solid-walled channel (i.e. no crevasse) revealed that thalweg bed-shear stress was approximately 2.86 N/m 2 for the middle reach. Consequently, given the assumption of a graded, near-equilibrium channel, I set the critical shear stress for erosion to 2.8 N/m 2 for these three classes. This value is similar to critical shear stress values for other cohesive bed rivers (Mier and Garcia, 2011). For non-cohesive sediments, Delft3D calculates transport automatically using the formulations of Van Rijn (1993) and grain diameter explicitly defines sediment classifications. Within this study, there is one classification for each sand size from very fine sand to coarse sand, and the modeled grain sizes for all classifications are shown in Table 6.2. In Delft3D, sediment-transport conditions are determined based on the proportion of sediments available in the channel bed; thus, designation of appropriate relative sediment thicknesses is necessary. Delft3D-QUICKEN was used to develop the three sets of depth files utilizing these 7 grain-size classifications for the fine, intermediate, and coarse channel distributions (Table 6.3). Model runs with fine grain-size distributions are 84.1% clay and finer 87

100 silt, middle grain-size distribution runs contain 80.1% coarse silt, very fine sand, and fine sand, and coarse grain-size distribution runs contain 84.1% medium and coarse sand (Figure 6.3). Within each distribution, the relationship between adjoining grain types is 2.5:1, such that there is 2.5 times as much clay as finer silt in the fine-grained channel distribution or vice versa for the intermediate-grained and coarse-grained channel distributions. All six runs contained the same floodplain bed-sediment, which was set to the fine channel grain-size distribution Initial and Boundary Conditions, Including Water and Sediment Discharge A series of set-up models helped determine parameters, initial conditions, and boundary conditions utilized in the final models. Three models one for each channel sediment distribution profile featured no crevasse or morphological changes for the duration of the simulation period, but included derived sediment thickness profiles from Table 6.3, which allowed equilibrium sediment concentrations to be characterized. This model was necessary because Delft3D-FLOW does not determine the equilibrium concentration profiles for cohesive sediments at model boundaries, so these boundary conditions must be user-defined. As Delft3D determines the equilibrium concentrations for non-cohesive sediments at model boundaries, values for the four sand classifications are not listed in Table 6.4 and were not user-defined in the final model runs. The set-up models were also used to determine the appropriate near-bankfull upstream water discharge (130 m 3 /s) and floodplain water-surface levels for the dry and wet model runs, 2.78 meters and 3.98 meters, respectively (Table 6.4). The normal depth (2.939 meters) corresponding to this discharge was determined via solving the following equation 3 H = C f Q 3 g S w 2 [7] with C f = f 8 [8] C 8g f and R 1/6 h n [9] where H is normal water depth, C f is a friction factor equal to the Darcy-Weisbach friction factor (f) divided by 8, Q is discharge, g is gravity, S is the channel friction slope, which is equal to 88

101 bedslope under uniform flow conditions (flow does not change from point to point at a given time interval), w is channel bankfull width, C is the Chezy friction factor, R h is the hydraulic radius, and n is the Manning friction factor. A typical Manning s n for straight sandy-bed rivers is 0.03 and this roughly equates to a Chezy friction factor of 43 m 1/2 /s (e.g. Julien, 2002; Chaundry, 2007). This Chezy value was used in all model simulations. To determine whether the designed channel was near equilibrium and graded conditions, three preliminary models utilized the sediment thickness profiles of Table 6.3, the derived sediment concentrations from Table 6.4, featured a solid-walled channel (i.e. no crevasse), 80x scaling factor, and morphological changes. This model was simulated for 45 days and the cumulative erosion and sedimentation profiles for the thalweg of the middle reach are presented in Figure 6.4. Over the 45 days, none of the three channels featured erosion or sedimentation in excess of 4 centimeters and the maximum difference is approximately 1.2% of the channel depth. Given the small magnitude of this variance, the model design is considered to be near equilibrium and graded conditions. Six additional set-up models one for each of the final models included the crevasse, sediment thickness files, and floodplain boundary conditions, but excluded morphological changes for the duration of the model. These models were used to determine the start-up conditions for each of the final run models, as these final runs featured a hot start. Additional user-defined input parameters are presented in Table 6.4. Of importance, the downstream channel boundary is a QH-relation to account for varying discharge (maximum value corresponds full discharge normal depth), which accounts for discharge loss through the levee crevasse. Also, each of the floodplain boundaries is either assigned a water-surface level (WSL) approximately 5 cm above the floodplain bed surface (2.78 meters for dry advective floodplain models) or ~20 centimeters below the water-surface of the solid-walled channel (3.98 meters for wet diffusive floodplain models), such that water entering the floodplain is effectively flowing into a lake with set water-levels. These conditions simulate high (dry) and low (wet) cross-floodplain water-surface gradients (e.g. Adams et al., 2004). Models were run at an 80x morphological scaling factor, which means each simulated days is equivalent to 80 days of morphological change. 89

102 6.2 Results Initial flow magnitudes (i.e. Day 0) in proximity to the crevasse are presented for the dry and wet model set-ups in Figures 6.5 and 6.6, respectively. For these two set-up conditions, the initial crevasse discharges are 24.4 m 3 /s and 10.1 m 3 /s, respectively, and the associated downstream discharges are m 3 /s and m 3 /s, respectively. The reduction in crevasse discharge exemplifies the importance of water-surface gradient in the delivery of water and sediment to the floodplain, as the bathymetry for these two model domains is otherwise identical. Throughcrevasse flow velocities are approximately 3.3 m/s and 0.7 m/s for the dry and wet models, respectively. Additionally, the turbulent jet propagating from the crevasse is directed in a more downstream direction for the diffusive transport (wet floodplain) run, and the expansion angle of this turbulent jet is much narrower, while the expansion angle of the advective transport (dry floodplain) run is nearly 45, which is the theoretical maximum (Machusick, 2000) Run 1: Fine Channel Sediment with Dry Floodplain Crevasse and downstream discharges (Figure 6.7) show a highly erosive initial phase as crevasse discharge increases to over 60 m 3 /s within a few simulation days and stays around this value for the duration of the model. Floodplain water flow is represented as a small lobate flow expansion (Figure 6.8) emerging from the crevasse mouth and features through crevasse velocities of ~ m/s. Following the 162 simulation days, significant erosion (~ 6 8 meters) occurred within the crevasse (Figure 6.9), but only limited deposition occurred elsewhere. Maximum deposition of meters occurred within two wings on either side of the crevasse mouth, and these wings may be attributable to deposition from diffusive eddies. Elsewhere, deposition greater than 0.4 meters presents as a bird s foot shaped pattern, and cross-sections (Figures ) reveal a deeply incised channel near the crevasse and generally gentle slopes towards splay margins. Deposition greater than 0.3 meters covers an area approximately 190,000 m 2, and deposition greater than 0.1 meters covers approximately 1,290,000 m 2 (Table 6.5). Sediment transport through the crevasse (Figures ) is extensive in the first 5 10 simulation days and eventually levels off to a rate of approximately m 3 /s, which is much less than the equilibrium rate observed in Run 1 s set-up model (Figure 6.14). 90

103 6.2.2 Run 2: Fine Channel Sediment with Wet Floodplain Over the day simulation, relatively little flow is diverted from the main channel (Figure 6.15), as there is a minor increase in through crevasse discharge to m 3 /s, which is roughly 10% of the upstream discharge. Final flow magnitudes show a slight downstream oriented zone of flow expansion with a narrow zone of higher velocities extending away from the crevasse opening (Figure 6.16). Through crevasse flow magnitudes are mostly m/s. Maximum erosion ( meters) occurs within and adjacent to the crevasse (Figure 6.17) and sediment deposition is limited (Figures ), with only 1 (non-channel) grid cell having deposition in excess of 0.3 meters and deposition greater than 0.1 meters limited to 24,250 m 2 (Table 6.5). Of the six model runs, sediment transport through the crevasse (Figures 6.12) is lowest for this scenario and the instantaneous sediment discharge (Figure 6.13) is less than m 3 /s. Normalized sediment discharge (Figure 6.14) is also lowest in this model run and is around 20% of the equivalent set-up model Run 3: Intermediate Channel Sediment with Dry Floodplain Run 3 crevasse discharges show an erosive initial phase, a brief quasi-equilibrium around 50 m 3 /s lasting until approximately until day 25, and finally an overall increase that persists until the end of the 198 day model run (Figure 6.20). At the end of the simulated period, the flow magnitudes (Figure 6.21) show the development of a distinct splay-channel bifurcation approximately 50 meters from the crevasse and two channels extending around 350 meters into the flood-basin. Flow velocities in these channels range from 0.7 m/s to 1.4 m/s. The splay channels eroded approximately 2 5 meters into the floodplain before the bifurcation and generally between 0.5 and 2 meters in the branches (Figure 6.22). Maximum deposition was between 0.8 and 1.0 meters in narrow bands surrounding the bifurcation, with a broad 300 x 250 meters area consisting of deposition between 0.6 and 0.8 meters. This broad area presents as a plateau on cross-sections (Figures ) and is surrounded by gentle slopes. The 0.3 meter contour encompasses an area of ~321,000 m 2 and the 0.1 meter contour an area of ~1,200,000 m 2 (Table 6.5). This model run features the highest cumulative sediment transport (Figure 6.12) and is the only run with a distinctly increasing instantaneous sediment 91

104 discharge (Figure 6.13). Normalized sediment discharge indicates that sediment discharge is greater in this simulation than in the hydrodynamic set-up model (Figure 6.14) Run 4: Intermediate Channel Sediment with Wet Floodplain Crevasse and downstream discharges show a sudden drop in discharge through the crevasse at the onset of modeling and then crevasse discharge around 6 m 3 /s for the duration of the model (162 days) (Figure 6.25). Final flow magnitudes in the crevasse s proximity are around m/s, and there is a rapid decline away from this plume with magnitudes dropping to less than 0.3 m/s within 50 meters (Figure 6.26). Net deposition of meters is found in a band extending from near the crevasse to approximately meters outward, which is further encircled by a thin meter band of sedimentation between 0.1 and 1.0 meters (Figure 6.27). Cross-sections (Figures ) show a plateau with steep, peripheral slip-faces. Thus, the transition from splay-associated deposits to flood-basin associated deposits is rapid, and the area encompassed by the 0.3 meter contour (~142,000 m 2 ) is only slightly smaller than the area encompassed by the 0.1 meter contour (~151,000 m 2 ) (Table 6.5). Cumulative total transport (Figure 6.12) is an order of magnitude less for Run 4 when compared to Run 3, and instantaneous sediment discharge is approximately m 3 /s, which is the second smallest value of any model run (Figure 6.13). Normalized sediment discharge is approximately 0.4, which indicates that sediment discharge is only around 40% of the value observed in the hydrodynamic set-up model (Figure 6.14) Run 5: Coarse Channel Sediment with Dry Floodplain Crevasse and downstream discharges (Figure 6.30) show the highly erosive initial phase with crevasse discharge increasing to greater than 50 m 3 /s, a mostly chaotic phase from Day 10 to Day 80, and a slow decline phase from Day 80 until the end of the simulation (Day 196) with final through crevasse discharge of approximately 41 m 3 /s. Flow magnitudes (Figure 6.31) show a similar pattern to Run 3, with a single channel extending approximately 80 meters out from the crevasse before a bifurcation, and these two secondary channels extending over 300 meters farther into the basin. Crevasse-channel flow velocities are approximately m/s in all of these channels. 92

105 Splay channel erosion is approximately 1 3 meters extending from the crevasse to the bifurcation and generally between 0 and 1 meter in the secondary channels (Figure 6.32). Deposition greater than 0.8 meters extends approximately meters out from the primary splay channel, and a broad (440 x 340 meters) area features deposition greater than 0.5 meters. Cross-sections C, D, and F (Figures ) capture this broad area of deposition and all cross-sections collectively show the gently sloping margins of the crevasse-splay. The 0.3 meter contour surrounds an area of ~315,000 m 2 and the 0.1 meter contour surrounds an area of ~1,160,000 m 2 (Table 6.5). Both areas are slightly smaller than the equivalent measurements for Run 3. The second highest values for cumulative sediment transport (Figure 6.12) and sediment discharge (Figure 6.13) are observed in Run 5. However, this run features the highest normalized sediment discharge indicating the greatest sediment discharge in excess of the baseline hydrodynamic set-up model Run 6: Coarse Channel Sediment with Wet Floodplain Crevasse and downstream discharges (Figure 6.35) indicate a brief depositional phase at the onset of the model followed by quasi-equilibrium with crevasse discharges of 4 7 m 3 /s for the duration of the model (141.5 Days). Flow magnitudes (Figure 6.36) are approximately m/s in a plume extending 40 meters outward from the crevasse and flow magnitudes greater than 0.3 meters are confined to an area extending meters outward from the crevasse. Similar patterns were also seen in Run 4 (Figure 6.26). Deposition patterns are also similar to Run 4 (Figure 6.27), with deposition between 1.0 and 1.2 meters extending approximately meters from the crevasse and this area surrounded by a thin 5 10 meters band with sediment thicknesses between 0.1 and 1.0 meters (Figure 6.37). Cross-sections (Figures 6.38 and 6.39) show this crevasse-splay as a plateau with steep slip-faces. Deposition greater than 0.3 meters covers an area of ~149,000 m 2, and deposition greater than 0.1 meters covers ~153,000 m 2 (Table 6.5), and both areas are slightly greater than the equivalent measurements in Run 4. Cumulative sediment transport (Figure 6.12) and sediment discharge (Figure 6.13) are also slightly greater than those of Run 4, with the latter value ~ m 3 /s. Interestingly, the normalized sediment discharge differs from Run 4 and is actually above 1, which indicates that more sediment is passing through the crevasse in this 93

106 model run than in its hydrodynamic set-up model (Figure 6.14). Along with Runs 3 and 5, these are the only models to feature a normalized value greater than Discussion These models give many insights into the influence of grain size and floodplain drainage on crevasse-splay development, and the parameter space in which small and large scale splay growth occurs. Systems dominated by clay and finer silt (i.e. Runs 1 and 2) show very little splay deposition overall both in surface area and sediment volume relative to equivalent intermediate and coarse-grained model runs (Table 6.5), and, consequently, also feature a lower throughcrevasse sediment discharge than runs with coarser grain sizes (Figures 6.12 and 6.13). There are differences between Runs 1 and 2, however. In Run 1, erosion dominates proximal crevasse settings, which is due to rapid crevasse flows and associated elevated shear stresses preventing fine sediments from falling out of suspension. The most extensive deposition presents as a bird s foot shaped wedge a few hundred meters from the crevasse. While crevasse flow velocity is lower in Run 2, the corresponding decrease in crevasse discharge limits sediment supply both from the main channel and eroded floodplain sediments leading to very limited deposition right at the crevasse (~ meters). All other model runs feature an equivalent amount of deposition hundreds of meters away from the crevasse throat. A modern system featuring finegrained small splay-like features is Red Creek in Wyoming (Schumann, 1989). Like model runs 1 and 2, splay deposition on Red Creek floodplains may be limited because the system lacks a sufficient supply of sediment that will settle from suspension on the proximal floodplain near levee crevasses. Outside of the fine-grained models, the primary control on crevasse-splay development appears to be floodplain inundation style. The wet intermediate and coarse-grained models (i.e. Runs 4 and 6) produce small splays extending approximately 300 meters outward from the crevasse with steep slip-faces on their splay margins. Splay area and volume calculations (Table 6.5) indicate these two models produce crevasse-splays that are similar in extent and basin-filling characteristics. These models also feature low crevasse discharges and immediate deposition within the crevasse at the start of the models. This suggests that highly inundated floodplains 94

107 suppress not only the basin-ward sediment advection, but through crevasse water transport as well. In contrast, splays produced in Runs 3 and 5, the dry floodplain runs, featured more expansive splay deposits and gently sloping profiles. Deposition greater than 0.3 meters and 0.1 meters extends approximately 400 meters and 750 meters, respectively, from the crevasse. In proximal settings, these splays are generally meters thick, which is thinner than in equivalent portions of the wet model runs. Unlike these wet model runs, there is distinct development of erosive splay channels in the crevasse and extending ~ 350 meters into the floodbasin with a bifurcation approximately 50 (Run 3) 80 (Run 5) meters from the crevasse. Maximum erosion extends well below the base of the primary channel and erosion declines along the length of the ~15 25 meter-wide splay channels. This erosive development phase is also captured in the sudden increase of through-crevasse discharge at the onset of modeling. Splay area and volume calculations (Table 6.5) show that these splays are much larger in area, and a greater volume of sediment is deposited in the region contained by the 0.1 meter contour. Within the region contained by the 0.3 meter contour, the sediment volume is nearly equivalent for Runs 3 6. The overall distributary pattern, gently sloping profiles, and more expansive scale are most similar to described splays in the Saskatchewan system (e.g. Smith and Perez-Arlucea, 1994; Perez-Arlucea and Smith, 1999; Farrell, 2001) and possibly the ancient Fort Union and Willwood Formations (e.g. Kraus and Wells, 1999). Ultimately, these six model runs suggest that overbank sedimentation via levee crevasses may be common as splay-like deposits were produced in all of the models, except Run 2 but that extensive basin-filling crevasse-splays may only occur in a rather limited parameter space. Expansive splays only occurred in well-drained systems featuring relatively abundant sand and large differences between channel and floodplain water-surface elevations. It s possible that in other settings, the appropriate grain sizes may shift. For example, fluvial systems in the upland Northern Pennines, United Kingdom produced splays featuring cobbles and boulders in a sand matrix (Table 1.1) (Macklin et al., 1992), but the river is substantially steeper. 95

108 Figure 6.1: Abbreviated modeling flow plan. Modeling of the three grain-size distributions will consist of two sets of computations comprising the same grid space and parameters except for exterior floodplain boundary conditions that will be varied to account for wet and dry floodplain conditions. 96

109 Figure 6.2: Modeled grid and bathymetry. (A) Upper figure shows the full grid. The long blue line at the bottom is the channel and the blue rectangle in the middle is the wider grid that includes the floodplain. Grid spacing is too fine to resolve much detail in channel or most of the floodplain. (B) The grey rectangle from A is the region in the crevasse s vicinity and is shown in the lower figure (B). In this area, grid spacing is 5 x 5 meters. 97

110 Table 6.1: Grid and Bathymetry Conditions. Domain Parameter Value(s) Channel and Levee Grid Cells in X Direction 321 Grid Cells Channel and Levee Grid Cells in Y Direction 15 Grid Cells Floodplain Grid Cells in X Direction 267 Floodplain Grid Cells in Y Direction 236 Range in X Grid Cell sizes meters Range in Y Grid Cell sizes meters Length of Grid in X Direction meters Length of Grid in Y Direction 2644 meters Area of Grid near Crevasse ( Wider Grid ) 3067 x 2634 meters Width of Channel-Levee Banks 5 meters (1 grid cell) Height of Channel-Levee (initial lip) Banks 3 meters Width of Levee-Floodplain Bank 5 meters (1 grid cell) Width of Levee Top 10 meters (2 grid cells) Width of Crevasse Opening at Levee Base/Top 5 meters (1 grid cell)/15 meters (3 grid cells) Approximate Distance to Crevasse Opening meters 98

111 Sediment Type Table 6.2: Modeled Sediment Fractions. Grain Type Corresponding Assigned Settling Grain Size Velocity (Cohesive) Assigned Critical Shear Stress (n/m 2 ) Cohesive Clay 1.95 Microns mm/s 2.8 Cohesive Very Fine-Fine Silt 7.81 Microns mm/s 2.8 Cohesive Medium-Coarse Silt Microns 0.88 mm/s 2.8 Non-Cohesive Very Fine Sand Microns Non-Cohesive Fine Sand Microns Non-Cohesive Medium Sand 375 Microns Non-Cohesive Coarse Sand 750 Microns

112 Table 6.3: Vertical Sediment Thicknesses of Modeled Grain-size Distributions. Grain Type Fine-Grained Distribution Models (meters) (Channel/Floodplain) Intermediate- Grained Distribution Models (meters) Coarse-Grained Distribution Models (meters) (Channel/Floodplain) (Channel/Floodplain) Clay 12 / / / 12 Very Fine-Fine Silt 4.8 / / / 4.8 Medium-Coarse Silt 1.92 / / / 1.92 Very Fine Sand / / / Fine Sand / / / Medium Sand / / / Coarse Sand / / / Cumulative Thickness / / /

113 Figure 6.3: Proportions of Each Sediment Fraction in Model Runs. This depicts the proportions of sediments in the channel beds for each model run. The fine channel fraction is the same as the floodplain bed fractions used in all of the models. Note that specific cohesive sediment fractions assigned for the upstream boundary condition are presented in Table 6.4 and that non-cohesive sediment flow fractions are determined within the model from these bed fractions. 101

114 Table 6.4: Final Model Parameters, Initial Conditions, and Boundary Conditions. These are the exact values changed from defaults or utilized in the final model runs. Parameter/Condition Advective Inundation Diffusive Inundation Model Length (days) Variable Variable Model Time Step (minutes) Surface Roughness (m 1/2 /s) Initial Hydrodynamic Conditions From Set-up Model Map File From Set-up Model Map File Sediment Effect on Fluid Density Yes Yes Morphological Scaling Factor 80x 80x Morphological Spin-up Time (hours) 0 0 Boundary Conditions Upstream Channel: Discharge (m 3 /s) Upstream Channel: Fine Model Clay: 5.6 x 10-3 Sediment Concentration (kg/m 3 ) Finer Silt: 1.9 x 10-3 Upstream Channel: Intermediate Model Sediment Concentration (kg/m 3 ) Upstream Channel: Coarse Model Sediment Concentration (kg/m 3 ) Coarser Silt: 1.9 x 10-4 Clay: 1.2 x 10-4 Finer Silt: 2.5 x 10-4 Coarser Silt: 1.6 x 10-4 Clay: 8.1 x 10-6 Finer Silt: 1.8 x 10-5 Coarser Silt: 1.1 x 10-5 Downstream Channel: QH Relation Minimum 0 m 3 /s : 0 meters (Discharge: Water-surface Level) Maximum 130 m 3 /s : meters 3 Floodplain Boundaries: WSL 2.78 meters (~5 cm above floodplain surface) Clay: 5.6 x 10-3 Finer Silt: 1.9 x 10-3 Coarser Silt: 1.9 x 10-4 Clay: 1.2 x 10-4 Finer Silt: 2.5 x 10-4 Coarser Silt: 1.6 x 10-4 Clay: 8.1 x 10-6 Finer Silt: 1.8 x 10-5 Coarser Silt: 1.1 x 10-5 Minimum 0 m 3 /s : 0 meters Maximum 130 m 3 /s : meters 3.98 meters (~20 cm below watersurface in channel without crevasse) 102

115 Figure 6.4: Cumulative Sedimentation in Channel with No Crevasse. This graph documents the cumulative erosion and sedimentation in the middle reach of the channel over the course of 45 simulated days. Given the small variance of channel bed elevation, this graph shows that the modeled channel is near equilibrium and graded. 103

116 Figure 6.5: Initial Flow Magnitudes for Dry Floodplain Models. The expansion angle for each side of the turbulent jet is nearly 45 from the primary flow direction, and flow is primarily directed away from the crevasse. 104

117 Figure 6.6: Initial Flow Magnitudes for Wet Floodplain Models. The expansion angle for each side of the turbulent jet is less than in the dry floodplain runs, and flow is directed in a more downstream direction than for the dry floodplain runs (Figure 6.5). 105

118 Figure 6.7: Crevasse and Downstream Discharges for Run

119 Figure 6.8: Final Flow Magnitudes (Day 162) for Run 1: Fine Channel Sediment with Dry Floodplain. 107

120 Figure 6.9: Cumulative Erosion/Sedimentation (Day 162) for Run 1: Fine Channel Sediment with Dry Floodplain. 108

121 Figure 6.10: Run 1: Cross-sections A, B, and C. 109

122 Figure 6.11: Run 1: Cross-sections D, E, and F. 110

123 Figure 6.12: Cumulative Total Sediment Transport Through Crevasse. These are the sediment volumes passed through a cross-section placed at the exit of the crevasse. Note that Delft3D calculates transport values ignoring the morphological scaling factor and assigned bulk sediment densities, thus this number is much lower than values based on simulated deposition (as shown in Table 6.5, for example). 111

124 Figure 6.13: Instantaneous Total Sediment Discharge Through Crevasse. These are the instantaneous sediment discharges passed through a cross-section placed at the exit of the crevasse. Note that Delft3D calculates transport values ignoring the morphological scaling factor and assigned bulk sediment densities, thus this number is much lower than values assumed simulated deposition (as shown in Table 6.5, for example). 112

125 Figure 6.14: Normalized Instantaneous Sediment Discharge Through Crevasse. These are the instantaneous sediment discharges normalized by the equilibrium instantaneous sediment discharge observed in preliminary set-up models used to determine the initial hydrodynamics for the final runs. 113

126 Table 6.5: Surface Areas and Volumes for Splays Produced within Models. Note that the following contour intervals include everything within (or beyond) that contour on the floodplain. Therefore, even areas within or near the crevasse that underwent erosion or saw deposition less than the given value are included in the > 0.3 and > 0.1 meter classifications. Also, Run 2 featured limited deposition and is listed with a maximum contour interval of 0.2 meters. Model Contour Interval # Grid Cells Surface Area (m 2 ) Volume (m 3 ) > 0.3 meter ,775 61,643 Run 1 > 0.1 meter ,291, ,730 < 0.1 meter ,556, ,400 > 0.2 meter Run 2 > 0.1 meter ,250 2,968 < 0.1 meter ,824,225 72,688 > 0.3 meter , ,637 Run 3 > 0.1 meter ,200, ,179 < 0.1 meter ,648, ,297 > 0.3 meter , ,962 Run 4 > 0.1 meter , ,446 < 0.1 meter ,697, > 0.3 meter , ,582 Run 5 > 0.1 meter ,156, ,042 < 0.1 meter ,691,620 97,416 > 0.3 meter , ,634 Run 6 > 0.1 meter , ,399 < 0.1 meter ,695,125 2,

127 Figure 6.15: Crevasse and Downstream Discharges for Run

128 Figure 6.16: Final Flow Magnitudes (Day 153.5) for Run 2: Fine Channel Sediment with Wet Floodplain. 116

129 Figure 6.17: Cumulative Erosion/Sedimentation (Day 153.5) for Run 2: Fine Channel Sediment with Wet Floodplain. 117

130 Figure 6.18: Run 2: Cross-sections A, B, and C. 118

131 Figure 6.19: Run 2: Cross-sections D, E, and F. 119

132 Figure 6.20: Crevasse and Downstream Discharges for Run

133 Figure 6.21: Final Flow Magnitudes (Day 198) for Run 3: Intermediate Channel Sediment with Dry Floodplain. 121

134 Figure 6.22: Cumulative Erosion/Sedimentation (Day 198) for Run 3: Intermediate Channel Sediment with Dry Floodplain. 122

135 Figure 6.23: Run 3: Cross-sections A,B, and C. 123

136 Figure 6.24: Run 3: Cross-sections D, E, and F. 124

137 Figure 6.25: Crevasse and Downstream Discharges for Run

138 Figure 6.26: Final Flow Magnitudes (Day 148) for Run 4: Intermediate Channel Sediment with Wet Floodplain. 126

139 Figure 6.27: Cumulative Erosion/Sedimentation (Day 148) for Run 4: Intermediate Channel Sediment with Wet Floodplain. 127

140 Figure 6.28: Run 4: Cross-sections A, B, and C. 128

141 Figure 6.29: Run 4: Cross-sections D, E, and F. 129

142 Figure 6.30: Crevasse and Downstream Discharges for Run

143 Figure 6.31: Final Flow Magnitudes (Day 196) for Run 5: Coarse Channel Sediment with Dry Floodplain.. 131

144 Figure 6.32: Cumulative Erosion/Sedimentation (Day 196) for Run 5: Coarse Channel Sediment with Dry Floodplain. 132

145 Figure 6.33: Run 5: Cross-sections A, B, and C. 133

146 Figure 6.34: Run 5: Cross-sections D, E, and F. 134

147 Figure 6.35: Crevasse and Downstream Discharges for Run

148 Figure 6.36: Final Flow Magnitudes (Day 141.5) for Run 6: Coarse Channel Sediment with Wet Floodplain. 136

149 Figure 6.37: Cumulative Erosion/Sedimentation (Day 75) for Run 6: Coarse Channel Sediment with Wet Floodplain.. 137

150 Figure 6.38: Run 6: Cross-sections A, B, and C. 138

151 Figure 6.39: Run 6: Cross-sections D, E, and F. 139