Physical History of the Platte River in Nebraska

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1 1 1 occurs in May or June. After the snowmelt peak, flow rapidly recedes to the lowest levels of the year during the summer. During summer months, the watershed can experience intense thunderstorm activity that can result in significant runoff although such events typically affect only a localized area. In contrast to the potential for large runoff events associated with snowmelt runoff or significant precipitation events, the Platte River also experiences extreme low flow periods and even periods of no flow. Multi -year droughts, such as the dust bowl era of the 1930's and the 1950's drought, provide an example of a series of dry years that resulted in very low annual and seasonal flows for extended periods of time. An analysis of water production (Simons & Associates, 1990e) showed that most of the water produced by the watershed comes from the mountains (based on gages located at elevations above 6000 feet). The data show that in the mountain region, annual water production from available gages ranged from 42 to 1704 acre -feet per square mile and averaged 484 acre -feet per square mile. In contrast, annual water production from plains region gages ranged from 6 to 116 acre -feet per square mile and averages 42 acre -feet per square mile. Thus, water production per unit area averages about 10 times more in the mountains than the plains. This distribution of water production coupled with climatic factors provided impetus for the development of the Platte River Basin. Rivers in the Platte River Basin cover a wide range of geomorphic characteristics as they convey water from the mountains over the alluvial plains. In the mountains, the first order tributaries flow over coarse beds consisting of gravel, cobbles, boulders, and bedrock outcrops. Flow is swift and turbulent but often carries little sediment due to the coarse and relatively stable nature of their river beds. Only on relatively rare occasions is the flow sufficiently strong to disrupt the channel beds and rework the coarse material forming these streams. As the streams flow out of the mountains and onto the plains, the streams do not have sufficient energy due to the decreased slope to transport the coarsest fractions of sediment being eroded and transported in the mountains. This sometimes results in coarse alluvial fans forming at the mouth of canyons or at least transition areas of coarse deposition with more dynamic behavior typical of alluvial streams. An alluvial river is one whose bed is formed of the material that the river itself transports. The river beds of the tributaries transition to considerably finer material consisting of sand and gravel. The bed of the two main tributaries (North Platte and South Platte) in the vicinity of their confluence that forms the Platte River consist primarily of sand. Sand -bed rivers, such as the Platte River in Central Nebraska, are truly alluvial channels that exhibit dynamic behavior responding to relatively frequent events. Since the EIS focuses primarily on the Platte River in Central Nebraska, most of the discussion will focus on the characteristics of the river in this region. When referring to the Platte River in the remainder of this report, the reference focuses on the Platte River in Central Nebraska unless discussion specifically shifts to another portion of the overall Platte River system. 3