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<br />western ranges, and its melting in June has a greater effect on the streams in that region than the
<br />melting of the snow over the eastern ranges has upon the streams draining the Front Range, In late
<br />spring, precipitation not only the Front Range, but also farther west sometimes augments the melting
<br />snow sufficiently to cause floods,
<br />
<br />3,3 Specific Basin Flooding Characteristics- Flood characteristics specific to particular major
<br />drainage basins are described below.
<br />
<br />:j,3,1 South Platte River
<br />
<br />Basin Descriotion- The headwaters of the South Platte River have their sources in the mountainous
<br />region surrounding the large basin near the center of the State and in the long eastern slopes 01 the
<br />high mountains forming the Continental Divide, The general course of the stream is eastward to lake
<br />George, then through Platte Canyon northward to its junction with the Cache la Poudre Flivm near
<br />Greeley, and then east again to its junction with the North Platte River at North Platte, Nebraska,
<br />
<br />Beyond Platte Canyon, the South Platte River emerges from the foothills and flows across the plains
<br />in a shallow valley for a distance of 190 miles to the Colorado-Nebraska State line, Through the
<br />canyon, the river has an average fall of 55 feet per mile, but across the plains, the river slope
<br />decreases from 15 to 7 feet per mile, The South Platte f~iver is rarely subject to floods above the
<br />canyon section, although, the general storm of June :2,'1, 1921, caused a flood that did serious
<br />damage.
<br />
<br />Floodina Characteristics, Below the canyon, the river is subject to floods, caused chiefly by the
<br />tributary streams draining the Front Range, and rarely by the tributaries from the plains area,
<br />Floodwaters on the South Platte River can result from intense localized rainfall or general rains, either
<br />of which may be augmented by snowmelt. The intense rainstorm floods result from intense rainfall
<br />over ,areas of relatively small areal extent. These storms can produce extremely rapid concentrations
<br />of floodwaters and ensuant rapid rises in stream levels, Because of little warning time prior to streams
<br />attaining flood 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 vicinity of the rainstorm, Tile
<br />discharge, however, attenuates rapidly on the South Platte R[ver downstream from the contributing
<br />tributaries due to valley storage of the floodwater,
<br />
<br />The general rainstorm floods result from rainfall over amas of large areal extent. Rainfall from this
<br />type of storm may occur over several days, Because of this time duration, the affected streams rise
<br />relatively slow; sometimes over a period of severa.l days, The duration of the flood period also is
<br />sometimes of considerable length, Because of the large areal extent of rainfall, more tributarios
<br />contribute to the peak discharge on the South Platte Fliver. This results in increasing (jischarge
<br />progressively downstream over a considerable length of the South Platte River. However, downstream
<br />from the contributing tributaries, the effects of valley storage cause flood discharge attenuation,
<br />
<br />Erosive effects can be considerable in the event of any flood regardless of its origin, Floods resulting
<br />
<br />'This Chapter taken from USGS "Floods in Colorado", 1948
<br />
<br />Colorado Flood
<br />llydrology Manual
<br />
<br />3,2
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<br />DRAFT
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