Abstract

The Platte River system of Nebraska is characterized by bankfull to overbankfull flow at high discharge with braiding occurring only around major islands. Linguoid bars dominate high flow and occur in both staggered and nested arrays similar in morphology to bars on the Tuna River, Norway. These lobate bedforms are active in the upper limits of the lower flow regime. The transition from high to low flow is characterized by braiding, which appears to be controlled by the high discharge bedforms. Depending on the rate of falling discharge, high-flow bars are modified to various degrees by active braid-channel erosion on their flanks; planation, dissection, and scour on their tops; and by bar-front progradation of multilobate, deltalike, lobes. Many of the modified bars (braid bars) bear little resemblance to their high-flow depositional geometry. Sets of large-scale planar cross-stratified coarse-grained sand up to 1 m in height in Platte River deposits record high-flow bars, whereas sets of large-scale planar and trough cross-stratified sand, and parallel laminated mudstone reflect intermediate and low flow braid channel and bar deposits. Extensive human modification of the Platte flow regime and the shallow depths of sampling trenches require that indirect inferences be made about the preservation potential of stratification produced by high, intermediate, and low flow bed configurations in the bed of the Platte. During the last 75 years, construction of dams and diversion canals has resulted in major decreases in channel width, increases in channel sinuosity, an overall flattening of the annual discharge curve, and an increase in arboreal vegetation in the channel and on the channel banks. These modifications to the river may have influenced the preservation of high, intermediate, and low flow stratification in the river beds, and therefore, sequences of stratification reported from the river bed may not fully exemplify sedimentation to be expected in ancient Platte-type braided rivers. Recognition of flow fluctuations may be the key to identification of ancient Platte-type braided rivers.

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