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Transverse Bars and Braiding in the Lower Platte River, Nebraska

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Transverse Bars and Braiding in the Lower Platte River, Nebraska NORMAN D. SMITH University of Illinois at Chicago Circle, Chicago, Illinois 60680 Transverse Bars and Braiding in the Lower Platte River, Nebraska ABSTRACT River in eastern Nebraska carries a dominantly The Platte is a wide, shallow river which sand bed load, and braiding is primarily flows eastward from the Rocky Mountains effected by dissection of transverse bars. across the Great Plains of Nebraska. Its lower In coarser streams, 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 channel into smaller topped transverse bars. branches as the flows recede and the mounds Transverse bars form by sediment 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 stream at some decrease on active bar 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 current direction, convex upward or dunes 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 Platte River 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 sediments, 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 sediment transport over served in the Red River (Waechter, 1970), the the entire bar surface. A single bar, examined Klaralven (Sundborg, 1956), the Loup Rivers closely over a five-day period of gradually de- (Brice, 1964, and this study), the Tana (Collin- creasing discharge, documents the evolution son, 1970), the Colorado River 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 Yellow River (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 braided river. 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/12/3407/3417657/i0016-7606-82-12-3407.pdf by guest on 25 September 2021 3408 N. D. SMITH—TRANSVERSE BARS, PLATTE RIVER, NEBRASKA mom area near highway crossings at North rection, bed material, bar configuration, and Bend, Fremont, and Valley, 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- Island, 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 tributaries, and principal study area. A Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/12/3407/3417657/i0016-7606-82-12-3407.pdf by guest on 25 September 2021 PHYSIOGRAPHY AND HYDROLOGY 3409 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 floodplain ranging in width from about 10 to 18 km and bound by low hills composed of various Pleistocene sedi- ments, mainly loess 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 flood-plain bank. Photographed July 11, 1970. portions of the central Rocky Mountains. Major tributaries of the lower Platte include the Loup Islands 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 spring 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 erosion from generally come in August and September. The the low-stage shifting anabranches, 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 thalweg winds FOR 5 PLATTE AND 2 Loup LOCALITIES through the bar-anabranch 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.
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