Physical History of the Platte River in Nebraska

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While there has been considerable analysis and discussion about the Platte River, significant disparity of opinion on a technical basis exists regarding the historic changes, factors that caused the changes, the significance of the changes, and potential mitigation strategies to deal with the affected environment. Because of the numerous issues and wide array of parties involved in one way or another in these issues (and nature of involvement), there appear to be significant sources of information that have not been widely available which may be useful in developing an understanding of the Platte River and related issues. The description of the Platte River is to be developed from a technical perspective. This technically -based description of the river would serve as a basis for a more general description of the affected environment for the Environmental Impact Statement (EIS) being prepared under the three -state agreement. The purpose of the technically -based description of the river is to: compile available technical information pertinent to the affected environment, present in condensed form the technical issues, present an evaluation of the techical issues, and discuss potential approaches to resolve these issues on a technical basis through the EIS process. This work will be guided by the concepts of using the "best available" information and application of the scientific method. The objective will be to produce an unbiased discussion by utilizing such information and by reaching scientifically justifiable conclusions or recommendations. 2.0 Description of the Platte River Basin The Platte River and its tributaries are formed as runoff from the Rocky Mountains east of the continental divide flow out of the mountains and over the plains towards the Missouri River (see Figure 2.1). The highest elevation in the Platte River Basin exceeds 14,000 feet above mean sea level and, as it joins with the Missouri River, the lowest elevation is just below 1,000 feet. The climate and hydrology of the Platte River Basin is as diverse as its topography. Precipitation is greatest in the mountains. In the area to the east of the mountains the precipitation is significantly less due to the rain - shadow effect of the mountains since storms generally move from west to east. As one travels farther east towards the Missouri River, precipitation gradually increases but remains less than that experienced in the mountains. In the mountains, with the generally colder climate at higher elevations, much of the annual precipitation falls as snow. Snowpack builds up over the fall and winter seasons with its release as runoff as the snow melts in spring and early summer. As a result of the interaction of climatic thermal regimes and precipitation patterns, the hydrologic cycle results in a relatively predictable seasonal pattern as indicated in Figure 2.2. Figure 2.2 presents an example of the flow hydrograph typical of the pattern exhibited by streams in the Platte River Basin. Starting in what is referred to as a "water- year," the flow is relatively steady from October through the winter months. As spring approaches there is a general rise in flow culminating in a snowmelt- generated peak flow that typically