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<br />8 <br /> <br />FACTORS INFLUENCING RUNOFF <br /> <br />There is a wide range of hydrological conditions <br /> <br /> <br />influencing runoff in the Upper Colorado River Basin. <br /> <br /> <br />The upper reaches are in the mount:ains where heavy <br /> <br /> <br />forest cover retards rainfall runoff. In the lower <br /> <br /> <br />portion of the basin, th,ere are a.reas of open pasture <br /> <br /> <br />and rangeland where rainfaLL runs off rapidly. The <br /> <br /> <br />highest peak runoff can :result when a spring rain storm <br /> <br /> <br />falls on snowpack. The ground is still partially frozen <br /> <br /> <br />so the water cannot infiltrate plus the rain melts some <br /> <br /> <br />of the snow to give a combined runoff. Normally the <br /> <br /> <br />magnitude of the peak flow depends only on the amount of <br /> <br /> <br />snowfall and the spring 1;hawing 1:empera1:ures. <br /> <br /> <br />Since 1937, eight reservoiJ:s have been built. in the <br /> <br /> <br />Upper Colorado River Basin. None were built for flood <br /> <br /> <br />control but they have affect:ed peak runoff th.rough normal <br /> <br /> <br />operation. In general the operators dra.w down the pool <br /> <br /> <br />level enough to hold the volume of spring runoff as pre- <br /> <br />dicted by the winter snow su.rveys, Limiting drawdown <br />factors in specific cases are minimum pool level for <br />hydroelectric operation, allowable pool level fluctua1:ion, <br />and wa1:er right requirements. 1'he eight reservoirs are: <br />