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During the winter, the moisture comes chiefly from the Pacific Ocean; it is deflected upward by the western mountain chains and is precipitated in the form of snow, usually heaviest on the western slope,s of the highest ranges and decreasing eastward, Along the Front Range, some precipitation, even in winter is caused by moisture from the Gulf. In general, however, the snowfadl is heaviest 0 the western ranges, and its melting in June has a greater effect on the streams in that region than the melting of the snow over the eastern ranges has upon the, streams draining the Front Range. In late spring, precipitation not only the Front Ranlle, but also farther west sometimes augments the melting snow sufficiently to cause :lIoods. 3.3 Specific Basin Flooding Characteristics- Flood characteristics specific to particular major dnllnage basins are described below. 3.3.1 South Platte River Basin Descriotion- '-he headwaters of the South Platte River have their sources in the mountainous region surrounding the large basin near the center of the State and in the long eastern slopes of the high mountains forming the Continental Divide. The general course of the stream is eastward to Lake George, then through Platte Canyon northward to its junction with the Cache la Poudre River near Greeley, and then east again to its junction with the North Platte River at North Platte, Nebraska. Beyond Platte Canyon, the South Platte River emerges from the foothills and flows across the plains in a shallow valley for a distance of 190 miles to the Colorado-Nebraska Stale line. Through the canyon, the river has an average fall of 55 feet per mile, but across the plains, the river slope decreases from 15 to 7 feet per mile. The South Platte River is rarely subject to floods above the canyon section, although, the general storm of June 2-7, 1921, caused a flood that did serious damage. Floodina Characteristics. Below the canyon, the river is subject to floods, caused chiefly by the tributary streams draining the Front Range, and rarely by the tributaries from the plains area. Floodwaters on the South Platte River can result from intense localized rainfall or general rains, either of which may be augmented by snowmelt. The intense rainstorm floods result from intense rainfall over -areas of relatively small areal extent. These storms can produce extremely rapid concentrations of floodwaters and ensuant rapid rises in stream levels, Because of little warning time prior to streams attaining flood stages, this type of storm can produce floods which are extremely hazardous. This type of flood produces high peak discharge on tributaries in the general vicinity of the rainstorm. The discharge, however, attenuates rapidly on the South Platte River downstream from the contributing tributarie,s due to valley storage of the floodwater. Colorado Flood Hydrology Mallual 3,2 a::w=r