THE CONVEX BAR: MEMBER OF THE ALLUVIAL CHANNEL SIDE-BAR CONTINUUM 1 CRAIG N. GOODWIN 2 AND JAMES R. STEIDTMANN Department of Geology University of Wyoming Laramie, WY 82071 ABSTRACT: Bars which form on the convex side of bends in Antelope Creek in eastern Wyoming superficially resemble point bars but have neither the genesis nor internal structure of true point bars in meandering perennial streams. Our observations of sediments in this shallow, sandy, ephemeral stream suggest that these bars are a function of "flashy" flow and large width-depth ratios which prohibit strong lateral flows related to helical cells. Apparently, sediment is transported parallel to downslream velocity vectors. During high flow, a large sediment mass is emplaced in the channel bend where flow expansion causes deposition. Further deposition takes place by vertical accretion as flows continue to overtop the bar. Subsequent low flows cause erosion of the bar by thalweg meandering and chute channel development, and deposition of fine material takes place in chute channels and on the bar margin. These processes result in the creation of a side bar with internal structure consisting of a main flood fill sediment body and vertically accreted topset beds with minor marginal foreset strata. A thin veneer of low-water fine sediment may occur on the outer bar margin and in chute channels. We suggest that these bars should be distinguished from true point bars and other types of side bars and herein propose the descriptive term convex bar. INTRODUCTION Point bar-like features which are structurally and genetically different from point bars in perennial, meandering streams form at bends in Antelope Creek, a sand-bottom ephermeral stream with "flashy" flow and large widthdepth ratio. These bars have neither the upward-fining grain-size changes nor the vertical sequence of sedimentary structures common to true point bars. Apparently a mechanism differing from that in meandering streams generates these features which we will call convex bars. The mechanics of point bar deposition in meandering streams is fairly well understood (Allen, 1965, 1970; Leopold and Wolman, 1960; Visher, 1965; Jackson, 1976). A helical flow cell is generated at a meander bend by currents that flow from the concave bank to the convex bank along the stream bottom and return along the surface, while simultaneously moving downstream. The lateral bottom current transports sediment to the convex bar SManuscript received February 14, 1980; revised June 16, 1980. 2Present address: Water Resources Research Institute, University of Wyoming, Laramie, WY 82071 where it is deposited as a point bar. This combination of flow and depositional conditions gives the classic point bar a generally upward-fining grain-size change and particular vertical sequence of sedimentary structures (Allen, 1970, Fig. 1; Visher, 1965, Fig. 13). Features at stream bends which are not classical point bars have been interpreted by several workers. In a study of Rio Puerto, an ephemeral stream in New Mexico, Shepherd (1976) notes that upward-fining grain-size changes and trough cross-stratification are rare in what he terms active point bars. He distinguishes four vertically sequential processstructure zones which, depending upon bend curvature and stage of development, comprise varying amounts of the bar. These zones are the thalweg, which conveys the lowest flow; the sheet bar, which is flooded yearly; the chute, which contains swale-fill structures; and the brush dune, which is bioturbated and has eolian deposits. Shepherd suggests that upperregime hydraulics and eolian processes play a major role in shaping these features. In a study of streams with coarse sediment loads, McGowen and Garner (1970) note features differing from those of point bars in streams with fine sediment loads. In what they define as coarse-grained point bars, upward- JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 51, NO. 1, MARCH, 1981, P. 0129--0136 Copyright 1981, The Society of Economic Paleontologists and Mineralogists 0022-a,:1.72/81/0051-0129/$03.00
130 CRAIG N. GOODWIN AND JAMES R. STEIDTMANN Fie3. l.--schematic diagram of a convex bar showing surfieial features (inner bar, chute channel, outer bar and channel) and internal structure {flood-fill, outer bar-chute channel and channel facies). fining grain-size changes are uncommon as is the typical point bar vertical sequence of stratification types. Most accretion of these point bars is attributed to extreme flood events. Streams characterized by this type of point bar carry dominantly bed-load sediments and have "flashy" flow, high gradients, and straight to meandering channel patterns. Hickin (1969) describes a process of point dune formation which he observed both in a flume and in the field. At supercritical flow, mobile dunes form in the channel, attach to the side, form an outer channel, and finally braid due to disequilibrium conditions (Hickin, 1969, Fig. 1). Although the external form of point dunes is similar to that of point bars, the internal stratification is entirely different in that the point dune is composed of foreset and topset strata. Point dunes are not lateral accretion features but appear to form in a manner similar to a Jopling (1965) delta. The terminology of Hickin (1969), McGowen and Garner (1970), and Shepherd (1976) is compared in Table 1. TAnLE I.-- Comparison of terminologies for channel bend features McGowan and Garner This Paper Hickin 0969) (1970) Shepherd (1976) Convex Bar Point Dune Point Bar Point Bar e~ Channel Scour Pool Thalweg Zone Channel Outer Bar Point Dune Lower Point Bar Sheet Bar Zone Inner Bar Chutes and chute Chute Zone (Chute channels) Dead Slough Zone fill, Chute Bars at Inner Bar Brash Dune Zone
ALLUVIAL CONVEX BARS 131 CHARACTERISTICS OF CONVEX BARS Convex bars were studied in Antelope Creek, a sand-bottom ephemeral stream in Converse County, Wyoming. Antelope Creek is 91 km long and drains an area of 2690 km 2. Flow is mainly the result of meltwater runoff and spring and summer thunderstorms, leaving the channel bottom subaerially exposed much of the year. The sinuosity of Antelope Creek varies from 1.18 to 1.25 and consists of irregularly spaced bends separated by straight and braided reaches. Braided reaches are located downstream from major bends where slumping of the concave bank supplies excessive amounts of sediment to the channel. Channel width varies from 25 m to over 200 m, and width-depth ratios calculated for bankfull conditions range from about 25 to over 125. The average slope of the channel is 0.0027 or 2.07 m/km (10.9 ft/mi). Mean sediment size in Antelope Creek is in the coarse sand range, but sediment from silt to pebble size is present. Surficial Features The physiographic features in channel bends of Antelope Creek are shown diagramatically in Figure I. A comparison of our terminology with that used by others to describe similar features elsewhere is given in Table 1. Concave banks are eroded into terrace deposits up to 20 m high. With the exception of the lowest terrace, concave banks are not flooded and therefore have no modern overbank deposits. Large quantities of sediment are contributed to the stream by erosion and caving of concave banks. FIG. 2.--Channel bend in Antelope Creek with convex bar, channel and cut bank in older terrace deposits. FIG. 3.--Erosional scarp at outer margin of convex bar where flow in channel has cut into bar sediments. The channel (Figs. 1, 2), which lies between the concave bank and the convex bar, is sandbottomed, fiat-bedded or tippled, and usually has less than 20 cm of relief. Sedimentary structures remaining on the dry channel bed include streaming lineation, scour holes, crescent scours, and ripples. The thalweg transmits the lowest flows, and its meandering within the channel creates pools and riffles. The pools collect fine sediment, support growth of algal mats, and often contain standing water long after the thalweg has ceased to flow. Riffles between thalweg pools are armored with a lag of pebbles. The convex bar surface is divided into inner (closest to the convex bank) and outer bar areas on the basis of sedimentary features. The inner bar is infrequently flooded and eolian effects are substantial. Coarse lag gravel armors the surface in places, and sand shadows occur on the lee side of vegetation. When emergent, the outer bar area is covered with a veneer of fine sand and silt a few centimeters thick displaying ripples and, less commonly, a plane bed or current lineation. Erosional scarps (Fig. 3) are created when flow in the channel erodes the outer bar area, and chute channels, lined with tippled fine sand and silt, form on top of the convex bar. Stratigraphy Three facies in channel-bend sediments are recognizable on the basis of grain size and stratification: the channel, flood-fill, and outer bar-chute channel facies (Fig. 1). For the most
132 CRAIG N. GOODW1N A ND JAMES R. STE1DTMA NN FIG. 4.--Sediments of the flood-fill facies showing horizontal stratification near the base and poor to massive bedding toward the top. part, convex bars are composed of sediments of the flood-fill and outer bar-chute channel facies. Deposits of the channels facies are 10 to 30 cm thick, lenticular in cross-section, and elongate parallel to flow. Mean grain size ranges from medium to coarse sand. Horizontal stratification or no apparent stratification is most common although a veneer of micro cross-stratification or climbing ripple stratification may cap the facies. The flood-fill facies comprises more than 80 percent of the sediments at a channel bend. These sediments consist of coarse sand, granules, and pebbles. Thickness of this facies is variable, difficult to determine, and a function of scour depth during flood flow. Sediments in the lower part are generally massive or flat bedded (Fig. 4). Toward the concave bank these sediments either intertongue with, or are overlain by, deposits of the channel facies. In the downstream direction and on the outer bar edge, these massive sediments give way to foreset bedding (Fig. 5). The upper part of the flood-fill facies is characterized by flat or lowangle topset stratification which covers the FIc. 5.--Foreset bedding at the downstream margin of a convex bar.
ALLUVIAL CONVEX BARS 133 underlying massive sediment and which grades upstream into the channel deposits of the preceeding straight reach. The outer bar-chute channel facies consists of fine sand and silt deposited during low flows on the outer part of the convex bar and in chute channels (Fig. I). The sediments accumulated on the outer bar are wedge-shaped in crosssection and appear similar to silt wedges depicted by Williams and Rust (1969, Fig. 19) on the sides of braid bars and the sand wedge shown by McKee et al. (1967, Fig. 12c). Sediments deposited in chutes are lenticular in cross-section. Micro cross-stratification and horizontal stratification are characteristics of this facies, which is volumetrically the least significant of the three facies identified in convex bars. FORMATIVE PROCESSES The distinct internal structure and texture of ephemeral stream convex bars suggests an origin which differs from that of perennial stream point bars. Perennial streams are characterized by fairly steady flow throughout the year with fluctuations for short intervals during floods. On the other hand, flow in ephemeral streams may range from insignificant to overbank depending upon drainage basin size, storm intensity, storm duration, and infiltration rate. For the creation of a convex bar and related physiographic features at a channel bend, both flood events, which provide a large volume of sediment from upstream sources, and low flow events, which rework this sediment mass, are necessary. During each distinct event the channel sediment is adjusted toward an equilibrium condition which differs from that developed during other flow intensities. The processes operating during both flood and low flow result in the specific surficial and internal structure of the convex bar. At channel bends of streams with large width-depth ratios lateral flow is suppressed or nonexistent because large helical flow cell generation is unlikely. Instead, it is likely that numerous, small, counteracting secondary currents result (Nemenyi, 1946). As a result, lateral sediment movement is minimal, and transport of most sediment is parallel to the downstream velocity vectors. This is significant in that the sediment mass which forms the convex bar is emplaced during flood flows when width-depth ratios at channel bends may exceed 100. Furthermore, streams with large width-depth ratios are generally uniformly shallow. Little depth variation from channel bottom to convex bar top results in inconsequential bedform changes. Point bars, on the other hand, show a variety of bedforms due to variation in water depth (Visher, 1965, Fig. 13; Jackson, 1976). Sedimentary Processes of Flood Flows Flood flows, although occurring much less frequently and for a shorter total time than low flows, supply the mass of sediment which forms the convex bar. During these periods when both competence and capacity are at a high level, a large quantity of sediment of all sizes is moved through the system. As flooding begins to subside, however, conditions promoting deposition are first encountered at channel bends where widening causes expansion of the flow. It is at this time that the main sediment mass of the convex bar is emplaced. As long as flood conditions are maintained, the bar continues to be overtopped and there is a tendency for it to build up to the water surface, a situation also reported by Hickin (1969) for point dunes. Continued flow expansion in the wider channel bend causes deposition in the form of lobate sheets which spread over the bar surface, and it is this vertical accretion which forms the topset strata found in the higher parts of the convex bar. Furthermore, because this vertical accretion deposition may override earlier-formed channel deposits, the convex bar commonly overlies or intertongues with channel deposits along its outer margin. Scour and fill of the channel produces discontinuous massive or graded bedding. The dissimilarities in cores taken only a few meters apart in straight reach channel beds attest to the lack of lateral continuity in these deposits. This observation supports Foley's (1976) contention that scour and fill does not occur simultaneously across the entire channel but rather that it is related to the migration of large-scale bed forms. At both the downstream end and outer edge of the convex bar, where there is a steep dropoff into the channel, sediment avalanching occurs during flood flow, and foreset strata are formed. The bar accretes very little by this process since rapid flow in the channel continually erodes these sediments while the flood is in progress. Foreset cross-strata are therefore of extremely limited extent and occur only
134 CRAIG N. GOODWIN AND JAMES R. STEIDTMANN marginal to the massive or flat-bedded main body of the bar in a setting analogous to the flood deposits described by McKee et al. (1967, p. 840). Sedimentary Processes of Low Flows Flows which do not overtop the convex bar tend to erode it as described by Miall (1977, p. 17). Since events of this type are much more frequent and of longer duration than flood flows, they significantly modify the flood flow sediments by leaving their own particular type of deposits and by eroding the flood deposits. During low flows the sediment transported and deposited is finer than in flood flows. Scour and fill takes place primarily in the channel and chute channels, and deposition of fine material occurs, to a limited extent, on the outer part of the convex bar. Erosion of convex bars during low flows occurs both by thalweg meandering and chute channel formation and enlargement. Where the thalweg makes contact with the outer margin of the bar, it erodes the bar sediments and may leave a scarp (Fig. 3). Depending on the frequency and duration of low flows, a major portion of the bar may be removed by this process. Chute channel development, as explained by Hickin (1969), occurs during the final stages of bar development when braiding takes place on the bar. As the stage falls, the chute channels cut into the bar almost to the level of the channel. Low flows enlarge the chute channels by lateral erosion. Sediment transport during low flows is by both traction and suspension. Sand-size sediments in the channel are moved over either a TABLE 2.-- Comparison of convex and point bar features Point Bar Convex Bar Small width-depth ratio Large width-depth ratio.~ Fine-grained bank Coarse-grained bank material material ~ Perennial flow Ephemeral flow w, ~ Meanders Channel bends ~.~ o~ Lateral accretion Vertical accretion Fines upward Coarsens and fines..~ upward discontinuously ~ Vertical sequence of Limited vertical se- ~ sedimentary structures quence of sedimentary "~ ~ structures plane or rippled bed, while fine sand and silt are transported in temporary suspension and settle out in areas of reduced velocity such as chute channels, shallows along edges of convex bars, and ponded water in channel lows. Eolian Effects During periods of low or no flow, wind removes sand from the surface of the bar and leaves a lag-gravel surface, and sand may accumulate on the lee sides of vegetation. These processes are mainly surficial and do very little to modify the convex bar stratigraphy. IMPLICATIONS The convex bars of Antelope Creek and true point bars are distinctly different, yet we believe that both are part of a channel-side bar continuum. This continuum of bar types is a function of the relative amount of lateral and vertical accretion on side bars, which is in turn related to periodic flow and generation of helical flow. In streams with small width.depth ratios, cross-channel sediment movement is important and typical laterally accreted point bars result. Where large width-depth ratios suppress lateral flow, the mainly vertically accreted convex bars result. A comparison of convex and point bar features is given in Table 2. It is quite likely that the convex bar is one end member of the side bar continuum in that with even larger width.depth ratios true braid bars rather than side bars result. This situation is approached when, during falling stage, the very shallow water over the convex bar begins braiding in the form of chute channels. Since a wide range of variation in the widthdepth ratio is possible, a number of side bar types, intermediate between the point and convex bar, are possible. These bars will have differing relative amounts of vertically and laterally accreted sediments. The side bars of the Amite River (McGowan and Garner, 1970) appear to be both vertically and laterally accreted, are associated with width-depth ratios around 40, and can probably be considered intermediate between the point and convex bars. Finally, there is probably some channel size above which actual depth, and not width-depth ratio, is the controlling factor. In very large streams width-depth ratio may be quite large, but depth is great enough to permit helical flow
ALLUVIAL CONVEX BARS 135 and lateral sediment transport. However, small streams with large width-depth ratios exert more bottom resistance to lateral flow, thus generating bars which are, for the most part, vertically accreted. This critical relationship between depth and width-depth ratio needs to be investigated further in studies of modern streams. CONCLUSIONS Antelope Creek, in eastern Wyoming, is a sand-bottom ephemeral stream with displays features at channel bends whose surficial form is nearly identical to point bars of perennial, meandering streams but whose internal structure and genesis are completely different. These features, which we call convex bars, form by vertical accretion and do not have the typical fining-upward stratigraphy and sequence of stratification types common to classic point bars. They do not result from lateral flow transporting sediment across the channel, but rather by vertical accretion when sediment is dropped from the flow at channel bends due to flow expansion and consequent decreasing velocities. We suggest that convex bars form in streams which are intermediate between typically meandering streams with point bars and braided streams. In the continuum of streams types, those with convex bars are at an endpoint for streams with attached side bars. Between the point bar and the convex bar channels in this continuum there are streams with side bars with varying amounts of vertically and laterally accreted sediments. The type of bar formed is probably most dependent upon width-depth ratio, at least for streams with relatively shallow depths. At width-depth ratios larger than those in streams with convex bars, side bars become detached and the channel is braided. ACKNOWLEDGMENTS We thank Leon E. Borgman for his help in this project. Financial support for this work came from Environmental Protection Agency Grant R803727-01-0 entitled "EPA Three State Study of Evaluation of Surface and Ground Water Problems Associated with Potential Strip Mine Sites." REFERENCES ALLEN, J. R. L., 1965, A review of the origin and characteristics of recent alluvial sediments: Sedimentology, v. 5, p. 89-191. --, 1970, A quantitative model of grain size and sedimentary structures in lateral deposits: Geological Jour., v. 7, p. 129-146. FOLEY, MICHAEL G., AND SHARP, ROBERT P., 1976, General scour and fill along a stream reach: Final Rep.. U.S. Army Res. Grant DAHCO4-74-G-0189, 8 p. HICK1N, E. J., 1969, A newly-identified process of point bar formation in natural streams: Am. Jour. Sci., v. 267, p. 999-1010 JACKSON, R. G., 1976, Depositional model of point bars in the lower Wabash River: Jour. Sed. Petrology, v. 46, p. 579-594. JOPLING, A. V., 1965, Laboratory study of the distribution of grain sizes in cross-bedded deposits, in Middleton, G. V., ed., Primary Sedimentary Structures and Their Hydrodynamic Interpretation: Soc. Econ. Palenntologists Mineralogists Spcc. Pub. No. 12, p. 53-65. LEOPOLD, L. B., AND WOLMAN, M. G., 1960, River meanders: Geol. Soc. Am. Bull., v. 71, p. 769-794. McGow~N, J. H., AND GARNEn, L. E., 1970, Physiographic features and stratification types of coarse-grained point bars: modern and ancient examples: Sedimentology, v. 14, p. 77 lll. MCKEE, E. D., CROSBV, E. J., AND BERRYmLL, H. L., 2g., 1967, Flood deposits, Bijou Creek, Colorado, June 1965: Jour. Sed. Petrology, v. 37, p. 829-851. MIALL, A. D., 1977, A review of the braided-river depositional environment: Earth Science Review, v. 13, p. 1-62. NEMENYt, P. F., 1946, Transportation of suspended sediment by water: A discussion: A.S.C.E. Trans., v. 111, p. 116-125. SrIEPHERD, R. G., 1976, Sedimentary processes and structures of ephemeral-stream point bars, Rio Puereo near Albuquerque, New Mexico (abs.): Geol. Soc. America Abs. with Programs, v. 8, p. 1103-1104. VISHER, G. S., 1965, Fluvial processes as interpreted from ancient and recent fluvial deposits, in Middleton, G. V., ed., Primary Sedimentary Structures and Their Hydrodynamic Interpretation: Soc. Econ. Paleontologists Mineralogists Spec. Pub. No. 12, p. 116-132. WILLIAMS, P. F., AND RUST, B. R., 1969: The sedimentology of a braided river: Jour. Sed. Petrology, v. 39, p. 649-679.