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Major changes in hydrologic regime and morphology of channels of the Platte River and its major tributaries, the South Platte River and North Platte River in Colorado, Wyoming, and Nebraska have occurred since about 1860, when the water resources of the basin began to be developed for agriculture, municipal, and industrial uses. The extent of this water development, which continues to increase with growth in population and land use, has affected the timing of streamflow and transport of fluvial sediment in the Platte River through diversions, reservoir storage, and increased use of groundwater. Changes in flow regime, such as increase in low-flow magnitudes and abatement of peak-flow magnitudes, have made the riverine environment conducive to vegetative growth while reducing channel scour. These factors, in turn, contribute to morphologic changes of decreased channel width and channel area and increased island formation. This paper will focus on these trends over the last several decades in the study area on the Platte River in Nebraska.

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Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material u m e coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution.