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and June. Following this springtime peak, streamflow decreases significantly during the <br />summer. In the fall, streamflow again increases to a relatively uniform level until the <br />following spring (Bentall and Others 1975). <br />Present -day streamflows, however, have been significantly modified by water <br />development activities within the basin. This hydrologic change has caused shifts in the low <br />and high levels of flow, and a flattening of the flow duration curve (Kircher and Karlinger <br />1981). These changes have also caused a significant decrease in the channel cross - sectional <br />area of the Platte River over the past 40 years, and an increase in island area (Kircher and <br />Karlinger 1981). <br />Groundwater levels within the study area are influenced by both the movement of <br />subsurface water down the Platte River valley, and the influence of regional groundwater <br />movements. The principal aquifer within the Platte River valley is formed by Pleistocene <br />sands and gravels (Schreurs and Rainwater 1956). The general direction of regional <br />groundwater movement is from northwest to southeast (Lugo and Wenzel 1938). Within the <br />Platte River valley, however, the direction of groundwater movement is almost parallel to the <br />direction of streamflow (Schreurs and Rainwater 1956). The mean groundwater gradient is <br />from 6 to 7 feet per mile, closely conforming to the average gradient of the Platte River <br />(Schreurs and Rainwater 1956). The water table within the Platte River valley is generally <br />shallow, and in the wet meadows the water table is usually at or near the ground surface. <br />Groundwater elevations are generally highest in the spring and lowest in the summer (Hurr <br />1983). <br />5 <br />