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<br />During the winter. the moisture comes chiefly from the PacWic Ocean; n is deflected upward <br />by the western mountain chains and is precipnated in the form of snow, usually heaviest <br />on the western slopes of the highest ranges and decreasing eastward. Along the Front <br />Range. some precipnation. even in winter is caused by moisture from the Gun, In general. <br />however. the snowfall is heaviest 0 the western ranges. and ns melting in June has a <br />greater effect on the streams in that region than the melting of the snow over the eastern <br />ranges has upon the streams draining the Front Range, In late spring, precipnation not <br />only the Front Range, but also farther west sometimes augments the melting snow <br />sufficiently to cause floods, <br /> <br />3.3 Specific Basin Flooding Characteristics. Flood characteristics specific to <br />particular major drainage basins are described below. <br /> <br />3,3,1 South Platte River <br /> <br />Basin Descriotion- The headwaters of the South Platte River have their sources in the <br />mountainous region surrounding the large basin near the center of the State and in the <br />long eastern slopes of the high mountains forming the Continental Divide, The general <br />course of the stream is eastward to Lake George, then through Platte Canyon northward <br />to ns junction wnh the Cache la Poudre River near Greeley, and then east again to its <br />junction wnh the North Platte River at North Platte, Nebraska, <br /> <br />Beyond Platte Canyon, the South Platte River emerges from the foothills and flows across <br />the plains in a shallow valley for a distance of 190 miles to the Colorado-Nebraska State <br />line, Through the canyon, the river has an average fall of 55 feet per mile, but across the <br />plains, the river slope decreases from 15 to 7 feet per mile, The South Platte River is <br />rarely subject to floods above the canyon section, although, the general storm of June 2-7, <br />1921, caused a flood that did serious damage, <br /> <br />Floodina Characteristics, Below the canyon, the river is subject to floods, caused chiefly <br />by the tributary streams draining the Front Flange, and rarely by the tributaries from the <br />plains area, Floodwaters on the South Platte River can result from intense localized <br />rainfall or general rains, either of which may be augmented by snowmelt, The intense <br />rainstorm floods result from intense rainfall over -areas of relatively small areal extent. <br />These storms can produce extremely rapid concentrations of floodwaters and ensuant <br />rapid rises in stream levels, Because of little warning time prior to streams attaining flood <br />stages. this type of storm can produce floods which are extremely hazardous, This type <br />of flood produces high peak discharge on tributaries in the general vicinny of the rainstorm, <br />The discharge, however, attenuates rapidly on the South Platte River downstream from the <br />contributing tributaries due to valley storage of the floodwater. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />3,2 <br /> <br />fFIJF( <br />