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2 <br />ANALYSES <br />To determine the range of potential effects from the Program's proposed environmental <br />water account, Reclamation analyzed recently collected data on rainfall, groundwater <br />levels, and riverflows which illustrate the effects of rainfall and river levels on ground <br />water levels in the valley. <br />Reclamation monitored 28 existing wells daily in four lines <br />across the Platte River (at Overton, Elm Creek, Minden, <br />and Alda), and compared daily readings from these wells <br />with three Platte River gauges and precipitation data from <br />March 11 through September 17, 1999. In the spring of <br />2000, monitors were installed in 16 of the wells to provide <br />supplementary data. Reclamation analyzed statistical <br />relationships among precipitation, riverflows and ground <br />water levels. <br />The USGS read all accessible wells within the Central <br />Conclusions are noted in <br />the sidebars for each <br />section. The text <br />provides rationales and <br />the appendices provide <br />the analyses and results <br />for these conclusions. <br />Platte River corridor to provide a snapshot of groundwater levels on May 25 -27, 1999. <br />Historic precipitation trends were analyzed to assess their probable influence on ground <br />water levels. A theoretical analysis also was done on the interaction between river levels <br />and ground water levels using Glover's Bank Storage equations. <br />HISTORIC SURFACE AND GROUND WATER REGIMES <br />The Platte River flow regime has changed since pre - development times, when the river <br />had large spring floods and often went dry in late summer. <br />Upstream dams and reservoirs, such as Kingsley Dam at Lake McConaughy and others <br />farther upstream, and other irrigation features have greatly reduced the spring flood's <br />magnitude and volume in the Central Platte. Simons (1999) reports "pre- development <br />conditions" (pre -1930) peak flows from 15,000 to 45,000 cfs, averaging 16,000 cfs. <br />Currently, peak flows are 3,000 to 24,000 cfs, with an average of 8,600 cfs. <br />Also, in more recent times, irrigation has created surface and ground water runoff to the <br />river throughout the summer. There has not been a no -flow month on the Platte River <br />since 1947. <br />Topographically, the Platte River lies at an elevation several hundred feet higher than the <br />Republican River some 40 miles to south of the Central Platte River and significantly <br />higher than the Big Blue and Little Blue Rivers to the southeast. In pre - development <br />times, ground water generally sloped from the Platte Valley toward these other streams. <br />Ground Water and River Flow Analyses <br />f <br />t <br />t <br />1 <br />1 <br />1 <br />1 <br />