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Home , Bar (river morphology), Braided river

NORMAN D. SMITH University of Illinois at Chicago Circle, Chicago, Illinois 60680 Transverse Bars and Braiding in the Lower Platte ,

ABSTRACT River in eastern Nebraska carries a dominantly The Platte is a wide, shallow river which , and braiding is primarily flows eastward from the Rocky Mountains effected by dissection of transverse bars. across the Great of Nebraska. Its lower In coarser , especially those with reaches carry a dominantly sandy load and dur- poorly sorted gravel beds, braiding is caused by ing intermediate and low discharges display a the construction during periods of high dis- pronounced braided character accomplished charges of low, linear, midchannel mounds. primarily through dissection of tabular, flat- These mounds divide the into smaller topped transverse bars. branches as the flows recede and the mounds Transverse bars form by aggrading become exposed. From field and laboratory ob- to a profile of equilibrium (Jopling, 1966) and servations, Leopold and Wolman (1957) grow by downcurrent extensions of avalanche showed that the formation of these longitudinal faces. Depth, velocity, and grain size tend to mounds requires only that a at some decrease on active surfaces from their up- point become unable to transport part of its stream mouths to the downstream and lateral coarsest load. The coarse sediment is deposited margins. Active surfaces are covered with and traps additional sediment causing the small-scale bed forms whose distributions are mound to build upward and in a downstream controlled by the flow characteristics. A typical direction. The resulting deposit is elongate in mouth-to-margin bed form progression is the direction, convex upward or to diminished dunes to ripples, reflecting slightly inclined on top, and it usually displays downcurrent reduction of stream power. Wa- a pronounced downstrean fining trend in sedi- ter-surface slopes over active bars tend to be ment grain size. These linear mounds, called greater than those of the channel segments longitudinal bars by Ore (1964) and Smith which feed them. (1970), dominate the upper reaches of the Under ideal conditions, transverse bars are South in Colorado (Smith, 1970) essentially lobate; however, most bars, espe- and in many other coarse bed streams (Fahnes- cially during low discharges, assume irregular tock, 1963; Doeglas, 1962; Krigstrom, 1962; or asymmetrical patterns due to any of several Ore, 1964; Williams and Rust, 1969). factors that include bar-mouth cross-sectional In braided streams composed of well-sorted geometry, proximity to exposed banks, adja- sandy , however, bars are more typi- cent currents, steadiness of flow, and basin cally transverse with wide, flat-topped tabular depth distribution. Braiding (bar dissection) bodies and sinuous to lobate depositional fronts begins during decreasing discharges when the which characterize the braided reaches of the flow passing through the bar mouth becomes lower Platte. Similar features have been ob- unable to sustain active over served in the Red River (Waechter, 1970), the the entire bar surface. A single bar, examined Klaralven (Sundborg, 1956), the Loup closely over a five-day period of gradually de- (Brice, 1964, and this study), the Tana (Collin- creasing , documents the evolution son, 1970), the in Texas from wholly active to dissected states. (McGowen and Garner, 1970), portions of the Rio Grande (Harms and Fahnestock, 1965; INTRODUCTION Culbertson and Scott, 1970) and probably the Since the early pioneers of the last century (Chien, 1961). first referred to it as "a mile wide and an inch The purpose of this report is to examine the deep," the Platte River has been one of the best processes and characteristic features associated known examples of a . Unlike with bar formation and braiding in the lower many braided streams, particularly those Platte, since details of these generally are lack- formed in glacial outwash plains, the Platte ing. Most of the data were obtained in the Fre- Geological Society of America Bulletin, v. 82, p. 3407-3420, 9 figs., December 1971 3407

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mom area near highway crossings at North rection, bed material, bar configuration, and Bend, Fremont, and , Nebraska, a dis- distribution of small-scale bed forms on bars. tance encompassing about 38 km. (Fig. 1). Ad- Velocities were determined by standard tech- ditional data were obtained further upstream niques using a pygmy-type Price current meter on the Platte at Schuyler, Chapman, and Grand because of the usual shallow depths encoun- , from the Middle Loup River at Danne- tered. Water-surface slopes were measured by brog, and from the Loup River north of leveling on a meter stick with a wooden base Palmer, Nebraska. The Middle Loup and Loup lowered to just touch the water surface. A large Rivers are smaller and have somewhat finer ring stand was usually adequate to hold the grained bed material than the lower Platte, but meter stick because of the predominately shal- otherwise they share many similar characteris- low depths. Surface current directions were re- tics. Most of the data were obtained at inter- corded from compass readings, and bed mediate to low discharges during July, early material was sampled by placing shallow pans in September, and late October 1970. Prelimi- stream-bottom depressions and allowing bed nary observations were made during the previ- forms to migrate over them. Stream-average ous two summers. samples for each locality were collected by split- ting a composite sample of 12-cm-long core METHODS specimens gathered at 15-m intervals in two Basic data included measurements of cross-channel traverses. Grain-size distributions velocity, depth, water-surface slope, current di- were determined by dry sieving. Plane table

NEBRASKA 0 100 KM.

OMAHA

Figure 1. Index map showing location of the Platte stream gauging station is located at North Bend. River, its major , and principal study area. A

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and alidade were used for mapping bar outlines and bed-form fields. A gauging station at North Bend provided stream discharge data which were made available by the Water Resources Division of the U.S. Geological Survey at Lin- coln, Nebraska. PHYSIOGRAPHY AND HYDROLOGY In the Fremont area of eastern Nebraska, the Plane River flows on a ranging in width from about 10 to 18 km and bound by low hills composed of various sedi- ments, mainly and till. The river drains an Figure 2. Typical braided reach of the lower Platte 2 near Valley. Forested areas in the left and right back- area of approximately 235,000 km in Ne- ground are, respectively, a permanent island and the east braska, Wyoming, and Colorado, including - . Photographed July 11, 1970. portions of the central Rocky Mountains. Major tributaries of the lower Platte include the Loup ranging from a few tens of meters to River at Columbus and farther downstream the 2 km in length and up to 400 m wide are scat- Elkhorn River north of Ashland. The area of tered throughout the Platte River. In 38 km concentrated study is located between the between North Bend and Valley, there are ap- mouths of these two tributaries (Fig. 1). In the proximately 65 islands. Their elevations are Fremont area, the Platte flows on a slope of similar to those of the nearby floodplain banks, about 1 m/km ~l. both of which are covered by mature cotton- Discharges are affected by seasonal climate wood forests and dense underbrush. Smaller changes, rainfall variations, power develop- grass-covered areas representing erosional rem- ments, storage reservoirs, and irrigation pro- nants of bars formed during high stages jects. Maximum discharges usually occur are abundant during the summer months. between March and June, and the lowest flows These grassy areas mildly resist from generally come in August and September. The the low-stage shifting , but rarely mean daily discharge recorded at North Bend survive the next year's spring discharges. from 1949 to 1969 is 109 m Vsec -l. The ex- The wide shallow nature of the lower Platte treme discharges for that same period are 3,172 results in a close correlation between air and mVsec-1 on March 29, I960, and 3.6 mV water temperatures. The extreme water tem- sec~' on August 29, 1955. peratures encountered in this study were 37° C The river bed between floodplain banks usu- on July 30 and 6° C on October 31. The river ally ranges from about 450 to 600 m wide in is usually frozen from early December to early the Fremont area. At high discharges, the active March. bed is completely covered and the river is not Channel-average median grain sizes for five braided. Braiding occurs at intermediate and Platte and two Loup localities are given in Ta- low flows when the river is choked with shifting ble 1. With the exception of the Middle Loup bars and shallow anabranches which vary con- at Dannebrog, the median channel-average siderably in number and size (Fig. 2). Fre- TABLE 1. CHANNEL-AVERAGE MEDIAN GRAIN SIZES quently at low discharges a main winds FOR 5 PLATTE AND 2 Loup LOCALITIES through the bar- complex. Width- /maximum depth ratios measured for 35 ran- Location Median Grain Size (mm) domly selected low-flow channels in late July averaged 97.6 and ranged from 36 to 341. Platte: During low summer discharges when braiding Grand Island .62 is best developed, channel depths seldom ex- Schuyler .40 N. Bend .43 ceed a few tens of centimeters, except where Fremont .38 the main thalweg flows against cohesive vege- Valley .33 tated banks; here, depths of up to 2 m may Middle Loup: occur. Many of the low-flow anabranches are Dannebrog .24 Loup: gently meandering with that rarely Palmer .30 exceed 1.4.

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grain sizes fall within the medium and coarse branches (similar to bars of Brush and sand range. A downstream fining tendency is others, 1966), and a variety of other solitary apparent from Grand Island to Valley, a dis- occurrences, including the convex sides of tance of 185 km. some channel bends. The latter have properties of Allen's (1968) transverse bars and the alter- TRANSVERSE BARS nating bars of Brush and others (1966), al- though the truly alternating forms tend to be General confined to narrow, fairly straight channels Transverse bars are the most characteristic with stable banks (see, for example, Maddock, and important bed forms in the lower Platte 1969, p. 60; Harms and Fahnestock, 1965) and River. They comprise a varied group of tabular, were not encountered in the lower Platte essentially flat-topped, periodic or solitary sand River. bodies which grow by downcurrent additions Transverse bars vary greatly in surface area, to slip faces (Fig. 3). Transverse bars, like from 1 or 2 sq m to many hundreds of square smaller scale bed forms, have been subject to meters. One asymmetrically lobe-shaped soli- nomenclatural inconsistencies. Brush and oth- tary bar mapped on the Loup River north of ers (1966) consider transverse bars to be soli- Palmer had an active surface area of over 3,700 tary or repetitive and to occupy nearly the sq m. Another active bar in the Platte River had entire width of a channel, whereas Allen an irregular but continuous slip face that ex- (1968) describes them as arranged along alter- tended over 200 m in length. Bar heights usu- nate banks of a channel. Culbertson and Scott ally range from a few centimeters to (1970) accept the former's definition for bars occasionally over 1 m. they studied in a Rio Grande conveyance chan- nel, but further regard them as equilibrium bed SURFACE BED FORMS forms transitional between lower flow regime The surfaces of active transverse bars are cov- dunes and upper regime plane beds. Sund- ered with small-scale bed forms which transport borg's (1956, p. 271) transverse bars are defi- sediment to the bar margins. These bed forms nitely repetitive and may be considerably less may vary from lower flow regime ripples to than full channel width. Most of the lower upper regime , depending on grain Platte bars, used here in the more general sense size and flow conditions over the bar surface. of Ore (1964) and Smith (1970), are not re- During the generally low discharges encoun- stricted to any of these criteria, but occur in a tered in this study, however, lower regime bed variety of shapes, positions in the channel, and forms were far more abundant than upper flow conditions. These include periodic or soli- regime forms. tary midchannel lobate forms (the linguoid bars On most bars, flows are deeper and swifter in of Allen, 1968, and Collinson, 1970), solitary upstream portions than in areas nearer the forms at the downstream ends of adjoining ana- depositional margins, and downcurrent progressions of changing bed forms are charac- teristic and frequent. For example, dunes with short curved crests, heights of up to 25 cm, and length/height ratios of < 15 usually occupy the deeper upstream portions of large bars. As the current shallows toward the downstream margins, the dunes may pass through a stage of diminished amplitude with wave lengths re- maining the same or even increasing, resulting in a series of bed forms with very low sloping stoss sides, amplitudes usually < 5 cm, and large wave length/amplitude ratios ranging from 20 to over 200. These "diminished dunes" then grade into linguoid ripples if depth and velocity decrease sufficiently toward Figure 3- Irregular downstream margin of active transverse bar showing well-defined slip faces and essen- the edge of the bar. Plane beds replace ripples tially flat-topped surface covered with ripples and plane where bed material exceeds 0.5 or 0.6 mm beds. Depth at 30-cm rule is 2 cm. median diameter.

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DUNES DIMINISHED DUNES a critical depth and velocity are once again at- TRANSITION, -DIMINISHED DUNE tained. The bar top thus becomes the new chan- RIPPLES PLANE BED nel floor over which sediment is once again transported. Jopling considers the deposit to be a "profile of equilibrium" which seeks to main- tain a critical depth necessary for transporting the available sediment. At high stream discharges, the mechanism of bar origin is less clear because observations are harder to make and local variations in depth and velocity are less obvious. Limited observa- tions during two periods of relatively high flows in early July and mid-September suggest that some bars, especially larger ones, begin as diffuse fields of dunes. Several such dune fields had vaguely defined frontal margins at high DIAMETER (MM.) flows: these margins became clearer as flows Figure 4. Relationship of low-regime bed forms to decreased. Apparently, the dunes at the down- stream power and median grain diameter. Allen's (1970) boundaries demarcating ripples, dunes, and lower phase stream margins of the fields coalesce as flows plane beds are included for comparison (dashed lines). begin to diminish and form straight, lobate, or Diminished dunes appear as a bed form distinctly separa- sinuous avalanche faces which define the trans- ble from ripples and dunes in stream power require- verse bar margins. Culbertson and Scott (1970) ments. suggest a similar origin for a transverse bar they Diminished dunes occupy a position inter- studied at high flow in a Rio Grande convey- mediate between dunes and ripples. Figure 4 ance channel. The hydraulic processes which represents stream power and median grain size initiate the local dune fields are not clear. data for 42 individual bed form fields on active Bars at high flows are often uniformly lobe- transverse bars. Stream power, a parameter shaped and sometimes assume a quasi repetitive thoroughly discussed by Bagnold (1966), is character, resembling the linguoid bars of defined as the product of bed shear stress and Allen (1968) and Collinson (1970). These average velocity and has been empirically were particularly well developed on the Loup shown by other workers (Simons and others, River during a mid-September discharge in- 1965, p. 52; Simons and Richardson, 1966, p. crease following a three-day rainy period. As 24; Allen, 1970, p. 314) to be highly useful for discharges decrease, however, they tend to coa- predicting occurrences of small-scale bed lesce and distort into a randomly distributed forms. variety of large and small solitary forms. Once formed, transverse bars have a pro- EVOLUTION OF TRANSVERSE BARS nounced effect on the local water-surface slope. The water-surface slopes were measured over Origin the full downstream lengths of eight active bars At low and intermediate discharges, trans- and over comparable lengths of the channel verse bars form in a general manner similar to immediately upstream from each bar. Without that described by Jopling (1966) for his exception the slopes were greater over the bars, "laboratory deltas." Shifting bars and anabran- sometimes exceeding the corresponding chan- ches and normal fluctuations in discharges nel slope by more than a factor of two (Table cause continual changes in local hydraulic con- 2). The steeper slopes tend to occur over bars ditions, thus variations in depth and velocity with the shallowest depths, a situation analo- over short distances are characteristic and fre- gous to the often observed phenomenon of quent. When sand moving along the stream abrupt slope increases over in coarse-bed bed encounters a depression, it will deposit if streams (Leopold and others, 1964). the depth increase is sufficient to lower the Once initiated, the bar enlarges by down- velocity below the critical value needed for stream and lateral extension of slip faces which traction transport. The sediment thus builds up define the perimeter of the bar. Sediment is from the floor of the depression in the cross- transported over the bar surface to its margins section form of a delta, constricting flow until by small-scale bed forms ranging from ripples

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TABLE 2. WATER SURFACE SLOPES OVER ACTIVE friction for the sediment population. Since TRANSVERSE BARS AND CHANNEL SEGMENTS stream power w increases with depth and IMMEDIATELY UPSTREAM FROM BARS velocity, the total sediment transport rates are greatest in the deep and swift portions of the No. Lb* Le* Sc* Sb/Sc* mouths of the bar. The deeper portions are 1 28.6 .0022 27.7 .0009 2.45 characterized by dunes, giving way to dimin- 2 32.6 .0021 30.8 .0015 1.40 ished dunes and ripples as depths decrease out- 3 22.2 .0012 22.0 .0011 1.09 ward. It is well known that sediment transport 4 18.9 .0022 18.0 .0021 1.05 by dunes is higher than for lower regime forms 5 29.0 .0018 24.1 .0009 2.00 6 10.5 .0030 18.1 .0011 2.73 (Guy and others, 1966; Williams, 1967). 7 32.0 .0010 26.2 .0007 1.43 If depth distribution is symmetrical so that 8 51.2 .0015 26.5 .0009 1.67 highest sediment transport rates are in the mid- dle of the bar and decrease gradually outward, "Lb, bar length in meters from mouth to downstream the bar may assume a symmetrical lobe shape, margin; Sb, water surface slope over bar; Le, length in meters of channel segment entering bar over which provided it is not modified by other factors slope was measured; Sc, water surface slope over channel (Fig. 8A). Where depth and transport rates are segment. asymmetrically distributed across the bar mouth, however, one side of the bar usually to antidunes, depending on flow conditions and grows faster than the other (Fig. 5A). sediment size. In the lower Platte and Loup Rivers, several factors determine the outline of Proximity to Stable Banks the bar (Fig. 5). These include cross-section Transverse bars expand both laterally and in shape of the bar mouth, proximity to stable a downstream direction. Lateral growth, how- banks, strength and direction of adjacent cur- ever, is arrested when the bar margin ap- rents flowing past the bar, steadiness of flow, proaches the stable banks of grassy bars, islands, and basin depth distribution. or the floodplain. In such cases, the migrating avalanche faces grow up to and parallel with the Cross Section of the Bar Mouth stable bank, maintaining a more or less constant The deepest and swiftest flow on an active distance from the bank (Fig. 5B). The width of bar is at the mouth or the upstream portion of the channel between the bar and bank is deter- the bar where the bed begins to slope upward. mined by the strength of flow over the bar In bilaterally symmetrical lobe-shaped bars, the edge; the bar will grow bankward so long as mouth usually occurs in the cross section be- currents restricted by the bar margin and the tween extreme upstream ends of the lateral bank are sufficiently weak to permit margins, or between one margin and the bank on the slip faces of the bar. As the bar-to-bank for some asymmetrical bars. All sediment trans- width narrows, the flow becomes more con- ported to the bar margins is carried through the stricted and eventually may become strong mouth, thus, the lateral distribution of sedi- enough to remove all sediment carried over the ment transport rates across the mouth affects bar edge. At this point, an equilibrium is at- the rates at which various sectors of the down- tained; the bar stops growing inward and a con- stream slip faces receive sediment. In active stant channel width between bar and bank is transverse bars, the cross-sectional depth profile maintained until flow over the bar changes. In of the mouth governs the relative rates of sedi- most cases, the width between bar and bank ment transport. Bagnold (1966) showed that increases slightly in the downstream direction total sediment transport rates should vary with to accommodate the additional flow added by stream power, the downstream portions of the bar (Fig. 5E). Adjacent Currents co^L- + o.Ol ^ tan a V Strong currents sweeping against the margins of the bar can modify the shape, remove any where;' is the rate of sediment transport, a) the sediment that is carried over the avalanche stream power, U the mean current velocity, V faces, and inhibit growth. The effects of adja- the fall velocity of suspended sediments, et a cent currents are most obvious at the junction bedload efficiency factor which depends partly of two anabranches: a bar is growing out of one on grain size, and tan a the coefficient of solid anabranch and is swept by the currents of the

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DEPTH CCNTOUB IN CM.

Figure 5. Plane-table map outlines of five transverse rection. D. Small lobes extend from earlier formed bar bars illustrating factors which affect bar shape. Details following decreased discharge and subsequent restric- discussed in text. A. Asymmetric lobe shape caused by tion of remaining flow over the bar surface; growth of greater sediment transport rates through the deeper por- top lobe is prevented by side currents. E. Bar grows tions of the asymmetric bar-mouth bottom profile. B. faster in shallow parts of basin near shore, even though Bar margin grows up to and parallel with exposed bank; more sediment is carried to deeper portions. Lower mar- growth of bar margin is determined by stability, shape, gin growth is restricted by exposed bank. Locations: A. and extent of the bank. C. Strong adjacent side currents Loup at Palmer, B. Platte at Fremont, C and E. Platte at sweep against bar margin to prevent growth in that di- Grand Island, D. Platte at North Bend.

other anabranch (Fig. 5C). Bar modification by lar observations were made by Brice (1964) on adjacent currents is most important during low transverse bars in the North Loup River. A discharges when variable current directions are subsequent depth increase will reactivate other best developed by anastomosing channels. portions of the bar, forming small-scale bed forms or one or more superimposed new trans- Steadiness of Flow verse bars. The final result may not be a single Transverse bars aggrade to a depth at which transverse bar, but rather a complex of smaller the flow is able to transport the available sedi- bars and bed forms which overlap, merge with, ment over its surface. If flow over the bar is or extend laterally from the edges of the origi- sufficiently reduced, parts of the bar become nal bar. inactive, and the current becomes confined to one or more small channels which continue to Basin Depth Distribution transport sediment to the bar margin. These Since transverse bars grow by accretion of small channels form new lobes as the sediment slip faces at the margins, they will tend to ex- spills over the edge of the bar (Fig. 5D). Simi- pand more quickly in a shallow basin than a

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deeper one. Downstream salients on the mar- decrease downstream and toward the margin in gins of irregular bars arise when the migrating surface grain size roughly corresponds to de- bar front encounters locally shallow portions of creases in stream power, a characteristic of vir- the basin. Bars with one margin near a bank tually all active transverse bars observed in this often grow more rapidly near the shore if the study. The effect of decreasing stream power basin is shallower there than in the deeper mid- on surface grain size is thought to be related to channel areas (Fig. 5E). the following three factors: A transverse bar will tend to assume a bilater- 1. In the stronger currents at the bar mouth, ally symmetrical lobe shape fanning away from more material is in suspension and less likely to the mouth if it has a symmetrical bar-mouth be trapped in the sample pans which collect bed depth profile and is uninfluenced by the other load sediments. As depth and velocity decrease, above factors. Such patterns are most com- some of the suspended sediment turns to bed monly attained during intermediate to high dis- load, giving a finer median size to the bottom charges, although they rarely persist for more sediments. than a few hours or days. The vast majority of 2. The efficiency factor for transport of active transverse bars observed in the lower coarse bed material is smaller than that for finer Platte River over the past three years have been sediment (Bagnold, 1966, Fig. 3): the finer bed either simple irregular and asymmetrical bars material is transported through the mouth and or complex bars that consist of two or more carried to the margins more quickly than the overlapping or laterally adjoining bars. coarser sediment, increasing the relative amounts of finer material trapped in the down- SURFACE GRAIN-SIZE stream sample pans. DISTRIBUTION 3. As flow power decreases, the coarser A characteristic feature of active transverse grains will be the first to deposit as a lag on the bars is a tendency for the bed material to bar surface, whereas finer material may con- become finer grained from the bar mouth to the tinue to move to the margins. This factor was margins (Fig. 6). A lobe-shaped bar that was especially noted on bars which had become sampled at North Bend displayed grain-size partly inactive due to diminishing flows. contours which were parallel to its outline; the The greatest grain-size differences between coarser material at the mouth became finer to- bar mouth and bar margin samples occur at ward the margins. As shown in Figure 7, the localities having the coarsest average bed load. For example, of the eight bars sampled in Fig- SAMPLE LOCALITIES: ure 6, the greatest difference occurred in two 1,2 - GRAND ISLAND bars at Grand Island, some 147 km upstream 3 - NORTH BEND 4 - FREMONT from North Bend, where the median cross 5,6,7,8 - PALMER (LOUP R.I channel grain size is approximately 0.62 mm. The smallest differences were observed on BED FORMS: Loup River bars north of Palmer, where the • DUNES median grain size is only 0.30 mm (Table 1). O DIMINISHED DUNES D RIPPLES BRAIDING A PLANE BED Braiding in the lower Platte River is a low discharge phenomenon brought about mainly by dissection of transverse bars. As discharge over an active bar decreases, local areas of the bar surface first become inactive, then exposed; the remaining flow becomes confined to one or more channels on the surface of the bar. A soli- tary bar studied over a five-day period under gradually diminishing discharges serves as a

25 50 75 typical illustration of bar dissection and deve- ^ bar mouth DISTANCE DOWNSTREAM (MJ lopment of braiding. The bar occurred near the Figure 6. Grain size changes on surfaces of eight ac- highway crossing west of Valley and was tive transverse bars from mouths to downstream mar- mapped at 24-hr intervals from July 8 to July gins. 12. During this period the mean discharge at

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I';.-::';:] EXPOSED AREA

''"••/»••.._ MEDIAN BED GRAIN DIAMETER (mm.)

— 500 STREAM POWER, tM (irgi/cm'/Mc) Figure 7. Plane-table map of active transverse bar at points: 24 for grain size, 57 for stream power. Effect of North Bend snowing contours of median bed grain size small bar emerging from bottom anabranch is negligi- and stream power. Both stream power and grain size ble. tend to decrease from the bar mouth to its margins. Data

North Bend decreased from 107.5 m3/sec~1 mouth discharges had decreased to 62.0 m3/ to 40.5 mVsec"1. Mean velocity, depth, cur- sec~* and 1.5 mVsec"1, respectively, and rent direction, and bed form type were re- the total bar area had increased to 2,101 m2 corded at 6 X 6 m intervals on the bar surface. (1,923 m2 active and 178 m2 inactive plus On July 8, the bar was essentially symmetri- exposed areas). Two more exposed areas had cally lobate and entirely active over its 1,709 appeared on the downstream margins, and the m2 surface (Fig. 8A). The bar mouth was well bar mouth had become more constricted, leav- defined as a cut through an inactive submerged ing exposed portions on both sides (Fig. 8B). bar; discharge through the mouth was 3.1 m3 Depth contours indicated that the region at the /sec ~!. Depth contours and bed-form field bar mouth was beginning to assume a - boundaries were symmetrically distributed and ing channel with a loop toward the west. more or less parallel to the bar margins. A mar- Velocity contours and boundaries ginward succession of dunes to diminished showed that most of the flow was down the bar dunes to ripples was well developed. Velocity toward the south, although some of the flow distribution was somewhat more irregular, but was divided by the small exposed areas on the the greatest velocities were clearly at the bar southwest margin, imparting a definite braided mouth. Currents emerging from the mouth aspect. Small lobe-shaped extensions at the spread radially over the bar surface. downstream margins resulted from slight ero- As discharges decreased the next day, a small sion and continual sediment transport between exposed area appeared at the southwest mar- the exposed areas. gin, and an irregular outline began to form. On the following day, the bar mouth dis- The following day, July 10, the river and bar charge decreased to 1.40 m3 /sec -1, and expo-

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Q= 107.5 m'/sec. Q= 62.O mVsec. = 3.1 mVsec. Qm = 1.5 m'/sec.

JULY 12

Q= 40.5 mVsec

Qm= 1.0 m'/sec.

Q RIVER DISCHARGE Qm BAR MOUTH DISCHARGE DEPTH CONTOUR (cm.) VELOCITY CONTOUR (cm./sec)

BAR EDGE, FORESETS BAR EDGE, DIFFUSE

CURRENT DIRECTION EXPOSED AREA BED FORMS: DUNES qn DIMINISHED DUNES RIPPLES Ui=J CU INACTIVE (within bar limits only)

Figure 8. Plane-table maps showing changes in a single bar over five days of gradually decreasing flows, shape, flow characteristics, and bed-form distribution for

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sures along the margins and on both sides of the mouths were readily defined were chosen, and mouth increased. The bar-mouth channel those having strongly irregular or asymmetrical became better defined and its shift toward the outlines were ignored so as to minimize the west more accentuated. On July 12, the river possible effects of uneven flow distribution and bar mouth discharges decreased to 40.5 over the bar surface. Bar-mouth discharges and 1.0 mVsec-1, respectively. By this time, were computed by close-spaced velocity and the original bar had become obscured and im- depth measurements. Active, exposed, and sub- possible to define as a distinct feature (Fig. 8C). merged inactive areas were distinguished by A small transverse bar growing from an adja- inspection of the bed: if mud was present or if cent anabranch had merged with and partly sand was not observed in transport, a sub- overgrown the eastern margin, and two new merged bed was considered inactive. smaller bars had begun to grow on the old bar The active and inactive areas and mouth dis- surface. Depth, velocity, and stream bed boun- charges for the 31 bars are plotted in Figure 9. daries indicated a clear westward meander loop A linear relationship appears to exist between in the bar-mouth channel. Another exposed bar-mouth discharge (Qm) and active surface area emerged just south of the bar mouth, and area on partially active bars (Aa). Below the flow coming through the mouth was divided line Aa = 352 Qm, the bar-mouth discharge is into three well-defined anabranches on the old sufficiently large for a given bar to maintain bar surface, although most of the flow con- active sediment transport over the entire sur- tinued to pass to the southern margin. Thus, face of the bar. As the ratio of surface area to what began as a single, well-defined lobate bar discharge increases by either decreasing the dis- had, in five days of steadily decreasing dis- charge, increasing bar area, or both, (in Fig. 8), charges, evolved into a complex of superim- an equilibrium discharge is eventually attained posed and laterally merged new transverse where the flow is just able to sustain active bed bars, irregular exposed areas, anabranches, and forms over the whole bar surface. Further bar shifting fields of small-scale bed forms. enlargement or discharge reduction then re- Over the next five days, discharges leveled sults in local inactive areas as the flow becomes off, but the "bar" continued to be modified. more confined. Eventually, the inactive areas Most of the current passing through the mouth become exposed as the flow becomes restricted became restricted to the meandering bar- to one or more channels which scour down- mouth channel, and additional smaller bars ward and laterally, marking the beginning of formed in the anabranches adjoining and flow- the braided pattern. ing over the old bar. On July 18, the discharge Figure 9 suggests that braiding dissection can once again began to decline, and all flow be accomplished without a reduction in bar- became confined to the now obviously mean- mouth discharge provided the bar-surface area dering bar-mouth channel. By July 24, when 9 WTLY ACTIVE MM, ACTIVE * INACTIVE PORTIWS 6 ACTIVE K*mc* OF PARTLY ACTIVE Uft (A.)

the average daily discharge for the river • WHOLLY ACTIVE BAR J dropped to 14.7 mVsec-1, flow in the bar area had completely stopped. Relationship of Bar-Mouth Discharge to Surface Area Examination of Figure 8 suggests that for a given transverse bar, there is a certain critical discharge passing through the bar mouth above which a wholly active sediment-transporting bar surface can be sustained. Below that critical discharge, inactive areas result, and dissection commences because the total flow is insuffi- ciently strong to maintain active sediment trans- port over the entire bar surface. BAR MOUTH DISCHARGE, Q. IM^SECJ To further investigate the relationship be- Figure 9. Relationship between bar-mouth discharge tween bar-mouth discharge and surface area, and active and inactive surface area for 31 transverse bars in the lower Platte and Loup Rivers. Regression line 31 active and partially exposed bars of varying represents the active surface areas of the partially inac- sizes were mapped. Only solitary bars whose tive bars.

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is permitted to expand beyond the critical value forms are fairly common during high dis- determined by Aa = 352 Qm. This possibility charges and during early stages of growth in could not be adequately tested because of rap- low discharges; ordinarily, however, the bars idly fluctuating local discharge conditions and take on a wide variety of asymmetric and ir- the tendency for bar growth to be interfered regular patterns soon after their initiation due with by adjacent currents and other nearby bars to one or more of the above factors. and exposed areas. Braiding begins when flow passing through Of added interest is the fact that the partly the bar mouth decreases to a point where it is inactive bars whose active portions are linearly unable to sustain sediment transport over the related to mouth discharge (Fig. 9) represent entire bar surface. The flow then becomes four localities whose median grain sizes range confined to one or more channels which begin from 0.24 mm (Middle Loup at Dannebrog) to to dissect the bar surface. During waning flow, 0.43 mm (Platte at North Bend), which sug- smaller transverse bars may merge with or gests tentatively that medium grain size is not of override the original bar. These new bars and overriding importance in the bar-mouth dis- their accompanying small-scale bed forms, com- charge-surface area relationship. It would seem bined with both downward and lateral dissec- that the slope of the line in Figure 9 should tion by surface and adjacent currents, result in decrease with increasing grain sizes entering a complex depositional and erosional history the bar mouth. for the original bar area after flows have dimin- ished or stopped completely. Thus, most ex- SUMMARY posed "bars," especially larger ones, separating Braiding in the sandy lower Platte River is an anabranches in the lower Platte River during intermediate and low discharge phenomenon low discharges are not simple exposed trans- effected primarily by dissection of transverse verse bars at all, but rather complex deposi- bars. Transverse bars form by aggrading to a tional and erosional features which merely profile of equilibrium and expand laterally by began as transverse bars. additions of sediment to avalanche faces. Bars formed during the high annual spring dis- ACKNOWLEDGMENTS charges are the first to become exposed. Those Financial assistance for this research was that escape complete erosion by waning cur- provided by a University of Illinois Summer rents are soon overgrown by vegetation to Faculty Fellowship and a grant from the Uni- become semipermanent features until de- versity of Illinois Research Board. I thank R. K. stroyed by next year's spring flows. Fahnestock, N. B. Waechter, S. A. Schumm, Dunes, diminished dunes, ripples, and plane and J. A. Campbell for their constructive com- beds are the principal small-scale bed forms that ments on earlier versions of the manuscript. F. occupy the surfaces of active transverse bars. J. Hein provided capable assistance in the field Depth, velocity, and stream power characteris- and laboratory. tically decrease from the bar mouth to its mar- gins, controlling the distribution of small-scale bed forms on the bar surface. A typical mouth- REFERENCES CITED to-downstream-margin sequence is dunes to di- minished dunes to ripples, with plane beds Allen, J.R.L., 1968, On the character and classifica- replacing ripples where median grain sizes ex- tion of bed forms: Geol. Mijnbouw, v. 47, p. ceed about 0.5 or 0.6 mm. Active bar surfaces 173-185. also display a definite grain size decrease from 1970, Studies in fluviatile sedimentation: a com- the mouth to the margins. parison of fining-upwards cyclothems, with spe- Several factors determine the shape of an cial reference to coarse-member composition evolving transverse bar. These include cross- and interpretation: Jour. Sed. Petrology, v. 40 sectional shape of the bar mouth, proximity to p. 298-323. stable banks, direction and strength of adjacent Bagnold, R. A., 1966, An approach to the sediment transport problem from general physics: U.S. currents, steadiness of flow, and depth distribu- Geol. Survey Prof. Paper 422-1, p. 1-37. tion of the floor over which the bar is growing. Brice, J. C, 1964, Channel patterns and terraces of The ideal bar pattern is a bilaterally symmetri- the Loup Rivers in Nebraska: U.S. Geol. Survey cal lobe shape with currents radially distributed Prof. Paper 422-D, p. 1-41. from the mouth over the surface. Such lobate Brush, L. M., Einstein, H. A., Simons, D. B., Vanoni,

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V. A., and Kennedy, J. F., 1966; Nomenclature Francisco, W. H. Freeman and Co., 522 p. for bed forms in alluvial channels: Am. Soc. Maddock, Thomas, 1969, The behavior of straight Civil Engineers Proc., Jour. Hydraulics Div., v. open channels with movable beds: U. S. Geol. 92, no. HY-3, p. 51-64. Survey Prof. Paper 622-A, p. 1-70. Chien, N., 1961, The braided stream of the lower McGowen, J. H., and Garner, L. E., 1970, Physio- Yellow River: Sci. Sinica, v. 10, p. 734-754. graphic features and stratification types of Collinson,;. D., 1970, of the Tana River, coarse-grained point bars: modern and ancient Norway: Geog. Annaler., v. 52A, p. 31-56. examples: Sedimentology, v. 14, p. 77-111. Culbertson, J. K., and Scott, C. H., 1970, Sandbar Ore, H. T., 1964, Some criteria for recognition of development and movement in an alluvial chan- braided stream deposits: Wyoming Univ. nel, Rio Grande near Bernardo, New Mexico: Contr. Geology, v. 3, p. 1-14. U. S. Geol. Survey Prof. Paper 700B, p. 237- Simons, D. B., and Richardson, E. V., 1966, Resist- 241. ance to flow in alluvial channals: U. S. Geol. Doeglas, D. J., 1962, The structure of sedimentary Survey Prof. Paper 422-J, p. 1-61. deposits of braided streams: Sedimentology, v. Simons, D. B., Richardson, E. V., and Nordin, C. F., 1, p. 167-190. 1965, generated by flow Fahnestock, R. K., 1963, Morphology and hy- in alluvial channels, in Middleton, G. V., ed., drology of a glacial stream, White River, Mount Primary sedimentary structures and their hydro- Ranier, : U.S. Geol. Survey Prof. dynamic interpretation: Soc. Econ. Paleontolo- Paper 422-A, p. 1-70. gists and Mineralogists Spec. Pub. 12, p. 34-52. Guy, H. P., Simons, D. B., and Richardson, E. V., Smith, N. D., 1970, The braided stream depositional 1966, Summary of alluvial channel data from environment: comparison of the Platte River experiments, 1956-1961: U. S. Geol. Sur- with some Silurian clastic rocks, north-central vey Prof. Paper 462-1, p. 1-96. Appalachians: Geol. Soc. America Bull., v. 81, Harms J. C., and Fahnestock, R. K., 1965, Stratifica- p. 2993-3014. tion, bed forms, and flow phenomena (with an Sundborg, A., 1956: The River Klaralven, A study example from the Rio Grande), in Middleton, of : Geog. Annaler, v. 38, p. G. V., ed., Primary sedimentary structures and 127-316. their hydrodynamic interpretation: Soc. Econ. Waechter, Noel B., 1970, Braided stream deposits Paleon,. and Mineral. Spec. Pub. 12, p. 84-115. of the Red River, Texas panhandle: Geol. Soc. Jopling, A. V., 1966, Some applications of theory America, Abs. with Programs (Ann. Mtg.), v. 2, and experiment to the study of bedding genesis: no. 7, p. 713. Sedimentology, v. 7, p. 71-102. Williams, G. P., 1967, Flume experiments on the Krigstrom, A., 1962, Geomorphological studies of transport of a coarse sand: U. S. Geol. Survey Sandur Plains and their braided rivers in Ice- Prof. Paper 562-B, p. 1-31. land: Geog. Annaler, v. 44, p. 328-346. Williams, P. F., and Rust, B. R., 1969, The sedimen- Leopold, L. B., and Wolman, M. G., 1957, River tology of a braided river: Jour. Sed. Petrology, channel patterns: braided, meandering and v. 39, p. 649-679. straight: U. S. Geol. Survey Prof. Paper 282-B, p. 39-85. Leopold, L. B., Wolman, M. G., and Miller, J. P., MANUSCRIPT RECEIVED BY THE SOCIETY MAY 13, 1971 1964, Fluvial processes in geomorphology: San REVISED MANUSCRIPT RECEIVED JULY 16, 1971

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