Physical History of the Platte River in Nebraska

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by over approximately 300,000 acre-feet annually. Thus, while the record may not indicate any significant changes due to water resources development, reductions inflow caused by increased consumptive uses in Colorado were at least partially offset by the transbasin import of water primarily from the Colorado River basin. 2.2.2 Peak Flow The peak flow hydrographs basically follow the same trend as exhibited by the total annual flow hydrographs (see Figures 2.14- 2.17). The gage on the North Platte River at North Platte, peak flows in the late 1800's and early 1900's, often exceeded 20,000 cfs and reached almost 30,000 cfs on one occasion. Peak flows dropped to a lower level after the initial construction of mainstem reservoirs on the North Platte River averaging about 10,000 to 15,000 cfs with a maximum of about 24,000 cfs in the time period from about 1908 to 1930. After the completion of several additional reservoirs in the 1930's - 1940's time frame, the peak flow on the North Platte River was dramatically reduced, rarely exceeding 5,000 cfs after 1940. A similar trend is apparent, although the period of record is shorter, at Overton on the Platte River. The same trend is not evident in the Julesburg record on the South Platte River, probably due to the influence of water resources development that occurred prior to the beginning of the historic record and the smaller amount of reservoir storage on the South Platte River compared to the North Platte River. 2.2.3 Seasonal Pattern of Daily Flow 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 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. In relatively recent times, 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. 16