The controls on and evolution of channel morphology of the Sacramento River: A case study of River Miles

Transcription

1 The controls on and evolution of channel morphology of the Sacramento River: A case study of River Miles Eric Larsen, Emily Anderson, Ellen Avery, Krishna Dole Geology Department University of California Davis, California Report to the Nature Conservancy

2 Table of Contents 1.0 Introduction Background Geologic Setting Hydrologic Setting Environmental Setting Historic Changes in Channel Location River Miles (Zone 1) River Miles (Zone 2) River Miles (Zone 3) River Miles (Zone 4) Methodology Historical Years of Record Modeling of Future Meander Migration Hydraulic Modeling of Flow Model Calibration and Validation Results Historical Years of Record Wavelength Sinuosity Area Reworked and Rate of Migration Modeling of Future Meander Migration Wavelength (Distance Between Inflection Points) Sinuosity Area Reworked and Rate of Migration References...

3 The controls on and evolution of channel morphology of the Sacramento River: A case study of River Miles Introduction River morphology as expressed in the planform shape, the cross-sectional shape, and longitudinal profile is the result of a complex interaction of the geologic setting, hydraulic factors, and environmental factors. As environmental managers are increasingly interested in managing river morphology in new ways that may enhance ecological functions, an understanding of what controls channel shape is critical. Intelligent decisions about what areas to manage, and how to manage them, depend on an understanding of how the controlling factors affect channel morphology and function. The Sacramento River is an example of a river where the channel morphology is determined by the interaction of geologic, hydraulic, and environmental factors. In this paper we use a selected study reach of 16 river miles (RM) (26 kilometers) as a case study to describe how these factors may have influenced historical channel characteristics, and how these factors can be used to inform future river management decisions. We examine the relative influence of each of these factors on channel morphology and meander migration pattern, and the interaction between them. We examine historical characteristics of the study reach, and predict future planform shape and location of the study reach under different rip-rap management scenarios. 2.0 Background 2.1 Geologic Setting The Sacramento River flows south through the Sacramento Valley over sedimentary rocks and recent alluvium. The Sacramento Valley is 96 kilometers (km) wide and 418 km long and is a structurally controlled basin between the Sierra Nevada Mountains and the Coast Range (Harwood and Helley 1987). The total drainage area of the river is 6.8x10 4 square kilometers (km 2 ), more than half of the total drainage area of the San Francisco Bay (Porterfield 1980). The location of the study reach is shown in Figure 1. The Sacramento Valley is located between the Sierra Nevada and the Coast Ranges of California. There are four major tectonic units in the Sacramento River watershed (Figure 2). The Coast Range, located on the west side of the river, is divided into the Great Valley sedimentary sequence on the east and the Franciscan formation on the west. The Klamath Mountains are an island arc terrane composed of marine sediments and granitic plutons, and are to the north and northwest of the Sacramento River. The page 1

4 southern Cascades and northern Sierran rocks are located to the west, and are areas of Pliocene-Recent extrusive volcanic activity. The valley that the river flows directly through is comprised of Pliocene-Pleistocene alluvium and fluvial deposits. The composition of sediments that are deposited into the Sacramento River Figure 1. Location of the Sacramento River and the study reach. from creeks is directly related to these tectonic units. For example, the bedload of Pine Creek, which meets the Sacramento River at RM 196 and drains from the east, is 89% volcanic clasts (Robertson 1987). The primary geologic units in the study reach are the Pleistocene-age fluvial (terrace) deposits of the Riverbank and Modesto Formations. These terrace deposits typically consist of 1-3 meters (m) of dark gray to red fine sand and silt overlying m of poorly sorted gravel (California Department of Water Resources [DWR] 1994). The Riverbank Formation is light red in color and consists of gravel, sand, silt and clay. There is soil formation in this unit that displays a B-horizon and local hardpan (DWR 1994). The Modesto formation is younger than the Riverbank formation, and contains the youngest terrace with a pedogenic B-horizon (DWR 1994). This unit is usually less than 2.5 m thick and is composed of gravel, sand, silt and clay (DWR 1994). The Riverbank and Modesto formations are generally erosion resistant, and when exposed on bends, these formations can inhibit bank erosion and lateral migration (Fischer 1994). Additionally, when these units are exposed on the downstream limb of a bend the page 2

5 downstream migration is limited. In the study reach, the west side of the river from RM 194 is constrained by the Modesto formation. There are paleochannel deposits located along the eastern margin of the Sacramento River from RM 226 (Thomes Creek) to RM 144 (Colusa) (Robertson 1987). The paleochannels are braided with multiple branches and islands, suggesting a higher bedload, a higher width-to-depth ratio, and higher discharges than the present day page 3

6 Figure 2. Northern California (Sacramento River watershed) with general structural regimes and two tectonic domains defined by Harwood and Helley (1987) within the Sacramento Valley region. Sacramento River (Robertson 1987). Based on buried soils, the age of the channels is between 150,000 and 450,000 years, equivalent in age to the Riverbank formation (Robertson 1987). There is a thin layer of silt and clay over the paleochannels, which has resulted in the paleochannels being mapped incorrectly as Modesto Formation in the past (Helley and Harwood 1985). The bedload of the paleochannels was significantly coarser than that of the present day Sacramento River and consisted primarily of volcanic rocks, most likely from a Cascade Range source area (Robertson 1987). This could be due to a period of high volcanic activity in the Lassen area. These paleochannels are wellindurated and erosion resistant, and the western edge of the paleochannel is the eastern edge of the historic meander belt of the present Sacramento River (Robertson 1987). The study reach is bound on the eastern side by paleochannel deposits from RM 193 to RM 185. page 4

7 The structural patterns of late Cenezoic deformation in the Sacramento Valley differs from the transpressional regime in the Coast Ranges to the west and the extensional deformation of the Basin and Range physiographic province to the east. The regional stress field in the valley has a maximum horizontal component of stress oriented eastwest (Harwood and Helley 1987). In the last 5.2 million years compressive deformation has progressed northward so that structures in the northern valley are younger than structures in the south. Harwood and Helley (1987) divided the valley into structural domains, and the ones of interest to this study are the Corning and the Chico domains, which are labeled in Figure 2. The Chico monocline is the dominant structure of the Chico domain (Figure 3). The Chico monocline and associated faults are the result of the uplift of the Sierra Nevada and fracturing along the major controlling fault (Harwood and Helley 1987). The Chico monocline trends northwest and bounds the northeast side of the Sacramento Valley between Chico and Red Bluff. The basement rocks beneath the monocline show a displacement of 350 m and there is evidence that the monocline is still active (Harwood and Helley 1987). West of the Chico domain is the Corning domain, with structures oriented northwest to north (Harwood and Helley 1987). The Willows fault and the Corning fault are within this domain, and they are close to parallel in orientation to the Chico monocline. The Willows fault is an active northwest trending fault that crosses the Sacramento River north of Colusa, with uplift to the east (Harwood and Helley 1987). The faults described above dominate the structure of the northern Sacramento Valley, however, the course and behavior of the Sacramento River is controlled by the smaller structures of the Los Molinos and Glenn synclines and the Corning Domes (Harwood and Helley 1987). Once the Sacramento River flows down the Los Molinos and Glenn syncline axes, the channel and floodplain of the river widens. For this particular study reach, RM , the river flows near the axis of the Glenn syncline. Upon entering the Glenn syncline at RM 205, the width of the channel and floodplain increases. The river is narrow from RM 200 to RM 197 as it crosses the axis of the syncline. The channel and floodplain widens again after RM 197, and the river parallels the axis of the Glenn syncline from RM 197 to RM 193. The channel crosses the axis of the syncline at RM 191, and then generally flows along the axis of the syncline until RM 180. There are two independent lines of evidence that supports the theory that these subsurface structures are still active: (1) a study of sediment deformation that shows recent movement on these controlling structural features (Helley and Jaworowski 1985), and (2) a National Geodetic Survey line that crosses the Sacramento River and the Glenn Syncline. Helley and Jaworowski (1985) studied the Red Bluff pediment (a gravelcovered erosion surface formed 450,000 years ago) and found that there was deformation of the erosional surface at the Corning Domes. They also mapped contours in the page 5

8 Modesto Formation (14,000-26,000 years old) that show depressions associated with the Glenn syncline. page 6

9 Figure 3. Structural map of the Sacramento Valley from Red Bluff to Colusa (from Harwood and Helley, 1982, after Fischer1994). The locations of flood relief structures and the National Geodetic Survey lines are indicated. page 7

10 More recent evidence of active deformation in the Sacramento Valley can be shown by the National Geodetic Survey (NGS) data (WET 1988). There is a line from Red Bluff to Roseville, California, that runs just to the east of the Sacramento River and crosses the Glenn Syncline. This line was surveyed in 1919 and in 1949, revealing subsidence to the south during this time interval. Additionally, there is a distinct disruption in the general trend where the survey line crosses the Glenn syncline, indicating a relative downwarping across the syncline (WET 1988). 2.2 Hydrologic Setting The major man-made structures that have affected the Sacramento River s hydrology in the last 60 years are the Shasta Dam and flood control structures installed as part of the federal flood control project of the Sacramento River. The hydrologic history of the Sacramento River within the study reach can be quantitatively assessed by accessing US Geological Survey (USGS) river gauge data. The USGS Hamilton City gauge (Number ) is located at approximately RM 199, near the top of the study reach. Daily flows were recorded from 21 April 1945 through 30 September Figure 4 shows a plot of annual peak flows. 2.E+05 1.E+05 1.E+05 Annual Peak Flow, Hamilton City Gauge, USGS No Flow (cfs) 1.E+05 8.E+04 6.E+04 4.E+04 2.E+04 0.E Water Year Figure 4. Peak annual flow, USGS gage no , Hamilton City gage. A return-interval analysis is performed for the study reach using Hamilton City gauge data from the USGS website ( The two-year return flow is calculated to be 79,500 cubic feet per second (cfs) (Appendix A). Discharge of 80,000 cfs is used in the meander migration modeling. page 8

11 Three major effects on discharge since the construction of Shasta Dam include: 1) a decrease in the minimum flow and an increase in the number of very low flows, 2) an increased occurrence of moderate flow throughout the year, particularly during the summer and fall irrigation seasons, and 3) a reduction in the number and volume of high and very high flows throughout the year (Buer et al. 1987). Regulation of discharge by Shasta Dam has increased summer flows from about 6200 cfs (for the years ) up to 10,520 cfs (for the years ) (Brice 1977). The maximum observed flood peak at Red Bluff before regulation was 250,000 cfs; since 1946, the maximum observed has been 140,000 cfs (Brice 1977). It has been observed that the period of 1946 to 1980 has experienced a 25% reduction in bank erosion rates from that of the 1896 to 1946 period (Brice 1977). This may be due to the reduction of high flows, which decreases bank erosion, or to the reduction in frequency and magnitude of peak flows due to Shasta Dam flow regulation (Buer et al. 1989). The design discharge for the federal flood control project of the Sacramento River south of RM 174 is 160,000 cfs at the Moulton Weir (RM 185) (USACE 1988). The design flow used for engineering the levee system is 300,000 cfs. Due to this constraint, there are three primary flood relief structures upstream of the levees at the following approximate locations (USACE 1988), which divert a total of 150,000 cfs upstream of Moulton Weir. 1. RM 191 (M & T Bend), diverting 70,000 cfs 2. RM (3B s, a natural overflow), diverting 35,000 cfs 3. RM 179 (Goose Lake), diverting 45,000 cfs Note that two of these structures are located within the study reach, capable of diverting up to 105,000 cfs. 2.3 Environmental Setting Before European settlement in the early 1800s, there was a wide strip of riparian forest along the Sacramento River (WET 1988). The first type of land converted to agriculture was known as rimland, which is adjacent to the river and at a higher elevation than the tule (swamp and overflow lands) in the basins (Buer et al. 1989). By 1871, almost all of this area was privately owned and being converted to agriculture (Buer et al. 1989). The floodplains were also progressively converted from riparian forest and tule swamp to agriculture, primarily fruit and nut orchards (Katibah 1984). By 1989, 98% of the original riparian forest was gone (Sacramento Handbook 2000). Micheli et al. (2002) compared migration of the Sacramento River for 50 years before and after the completion of Shasta Dam, and found that despite flow regulation, bank migration rates and erodibility increased approximately 50% as riparian floodplains were progressively converted to agriculture. Agricultural floodplains are % more erodible than riparian forest floodplains (Micheli et al. 2002). Brice (1977) maintains that riparian vegetation promotes higher sinuosity because it can inhibit chute cutoffs, page 9

12 therefore clearing trees would result in a reduction of sinuosity and a change in channel form. There are cases where even a narrow fringe of riparian vegetation can deflect the flow of the river and prevent rapid bank erosion (Brice 1977). Also, riparian vegetation on the inside of the meander loop inhibits downstream migration of the meander loop and helps prevent cutoffs (Brice 1977). Micheli et al. (2002) note that bank strength may be increased by root reinforcements in riparian-rich areas. Channel roughness is higher for riparian forest border reaches than agriculture rich reaches (Micheli et al. 2002). If meander migration mirrors the flow field, vegetation removal could cause greater migration in the downstream versus the cross-stream direction and result in perhaps a less sinuous channel pattern over time. 2.4 Historic Changes in Channel Location The factors described above geologic controls, hydrological changes and changes in land use have a combined affect on the location of the present day Sacramento River. The largest influence on channel location and meander migration in the study reach has been the progressive installation of riprap through time. Additionally, the amplitude of bends within the study reach has decreased through time. The four separate parts of the study reach described below exhibit wide variation in meander migration rates, sinuosity, and type of vegetation on the banks. We describe channel movement in the study reach of the Sacramento River from 1870 to 1997, using banklines mapped by Greco, et al. (2002). Channel bank locations span approximately 100 years ( ) and, depending on data availability, were mapped at intervals varying from one to 17 years. For years prior to aerial photography (1870, 1887, 1904, 1920), channel bank locations were chosen to coincide with channel edges indicated on USGS topographic maps (1:68,500). The USGS method for defining these channel banks is undocumented. Where aerial photography is available (1937 through 1997), bank locations were mapped from air photos ranging in scale from 1:40,000 to 1:6,000. The majority of photos were flown at a scale of 1:10,000 or smaller, allowing for high mapping precision. Banklines were mapped by tracing the contact between river water and adjacent dry land and these traces were then rectified in the Arc View/Arc Info GIS environment. For a detailed description of mapping and rectification methods, see Greco et al. (2002). The study reach is divided into four main subreaches, or Zones: RM (Zone 1), RM (Zone 2), RM (Zone 3), and RM (Zone 4). Banklines are shown with the older years overlain by more recent years for all figures in this section. We describe the study reach starting upstream and moving downstream. River Miles (Zone 1) Since 1904, this reach has been characterized by channel stability. The bend at RM 199 narrowed between 1870 and 1887, and cutoff by 1904 (Figure 5A). The channel configuration after the cutoff establishes the river left bend at RM 198 that still exists page 10

13 today. The reach from RM has been very straight with little or no shift of the channel after the cutoff of the bend at RM 199 in The channel width has also been relatively constant since 1904 (Figure 5B). There is no obvious geological control on the channel, and riprap was installed by However, this is the location of the axis of the Glenn Syncline (discussed in Section 2.1 Geologic Setting). The full effects of the Glenn Syncline on the Sacramento River are not known, but evidence suggests that channel location and shape is influenced by this subsurface structure. The apex of the bend at RM 198 migrated downstream until 1975, at which time rip-rap was installed along the outer bank (Figure 5B). Figure 5. River miles Historical river channel movement from (A) and(b) River Miles (Zone 2) The reach from RM encompasses two large bends of interest the bend near the confluence of Pine Creek and the Jenny Lind Bend. The Pine Creek Bend has been quite mobile through time. In the reach that contained the Jenny Lind Bend, RM , there has been a decrease in the amplitude of the meander bends, but the wavelength has remained relatively constant. The Pine Creek Bend (RM 199), becomes established after a cutoff by The confluence of Pine Creek and the Sacramento River migrates east between the late 1800s and 1904 this is as far east as the river channel ever moves (Figure 6A). Pine Creek Bend migrates downstream over the years. The main channel is abandoned and a secondary channel, or cutoff channel, is occupied by 1980 (Figure 6C). The river essentially does not migrate again in this area. Between 1870 and 1920, the Jenny Lind Bend maintained high sinuosity and migrated downstream. By 1937 the Jenny Lind Bend experiences a cutoff (Figure 7B). This subreach of the river experiences small movement and sinuosity changes. Riprap was installed between 1974 and 1980, limiting meander migration. page 11

14 Figure 6. Pine Creek Bend, RM Historical channel movement from (A) , (B) , and (C) page 12

15 Figure 7. Jenny Lind Bend, RM Historical channel movement from (A) , (B) , (C) , (D) , (E) , (F) , and (G) page 13

16 Figure 7 continued. River Miles (Zone 3) This area on the river (RM ) is the least geologically constrained, and the river migrates freely in this location prior to the installation of riprap, exhibiting classic meander bend forms. From the bends in this area migrate downstream and constrict (Figure 8A). There is no data for 1920, and by 1937 the bends are very different in location and shape (Figure 8B). What is clear is there is an overall decrease in page 14

17 sinuosity caused by two cutoffs, one at RM 190 and another at RM (Figure 8B). From , there is downstream migration of the bends, and by 1974 the channel has migrated or cutoff into a new channel (Figure 8C). After 1974, the bends continue migrate downstream, especially at RM 192 and RM 190 (Figure 8D). The bends are constrained by riprap, but exact dates for the riprap installation are unknown. There is a flood control structure (M&T) at RM Figure 8. River Miles Historical channel movement from (A) , (B) , (C) , (D) and River Miles (Zone 4) Over the time of historical record, this subreach (RM ) has developed two distinct and large bends. Monroeville Bend (RM ) develops into a very high amplitude bend that been constrained in its current location by riprap. Kimmelshue Bend (RM ) progressively develops into a single bend that then becomes a bilobate bend (compound bend consisting of more than one identifiable radius of curvature) where one of the smaller bends cuts off. Monroeville bend is present during the earliest year for which we have data (1870), and by 1952 it has migrated downstream and rotated its page 15

18 orientation to very close to its current location (Figure 9A and 9B). By 1978 riprap was installed, preventing what appears to be a potential cutoff, and the bend is essentially static until 1987 (Figure 9E). In 1978, there was the development of a small side channel at approximately RM 190, and the river eventually occupies this side channel as the main channel in 1987 (Figure 9D and 9E). Kimmelshue bend is established in 1904 (Figure 9A). In 1974, the bilobate character of this bend becomes very pronounced, with one lobe bounded by inflection points at RM , and the other from RM (Figure 9C). By 1987 the downstream lobe has cutoff (Figure 9D and 9E). This cutoff changed the orientation of the downstream bends at RM In this reach, there are private diversion structures at RM 188.5, 187, 185 (two) and 184. The primary use of the water from these small pumps is for agricultural irrigation. page 16

19 Figure 9. Monroeville and Kimmelshue bends, RM Historical channel movement from (A) , (B) , (C) , (D) , (E) , and (F) page 17

20 Figure 9 continued. 3.0 Methodology 3.1 Historical Years of Record Changes of the four subreaches, or Zones, within the study reach were quantitatively analyzed using available historic data. Figure 10 shows the location of the Zones and individual bends in 1904, the earliest year of record used in this analysis. We present graphical results for the evaluation of each zone for the following variables: 1. Distance between inflection points (wavelength) 2. Sinuosity 3. Area reworked and rate of bend migration The 1870 and 1887 historic years of record are not included in the quantitative analysis, primarily because planform data for these years are unreliable. Planform data for these years of record originated from topographic maps that, when digitized and rectified, located the Sacramento River in an unlikely area. For the discussion in Section 2.4 Historic Changes in Channel Location, these years were included; however the location of the river was manipulated to agree with later years of record. There are cases where structural elements of the river are referred to as bends, although they may be almost straight in a given year. This was done because in other years they may develop significant curvature, and accounting for the whole river as bends page 18