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of 19,800 in 1980 and 18,852 in 1970, an increase of 5 percent (Reference 4). The climate of Logan County is primarily affected by a northwest wind. It has an average annual precipitation of 19 inches, not including an average annual snowfall of 30.4 inches. The mean annual temperature is 48.60F. The coldest month is January, with a mean temperature of 38.80Ft and the warmest month is July, wi th an average temperature of 89. 90F. The average annual number of clear days is 299. The total land area for Logan County is 1,849 square miles and is mostly comprised of high rolling plains. The land is used primarily for the production of livestock grains, hay, and oil products. 2.3 Principal Flood Problems Two crossings obstruct floodflows along the South Platte River in Logan County. These are the Burl ington Northern Rail road and U.S. Highway 6 bridges. The Pawnee Creek Overflow is created by the U. S. Highway 6 and the combined Union Pacific Railroad and the Burlington Northern Railroad bridges crossing Pawnee Creek. During periods of high stage on Pawnee Creek, floodwaters can escape the Pawnee Creek valley to flow over land near Riverside Cemetery toward the City of Sterling due to the constrictive crossings. Flooding along the South Platte River and Pawnee Creek Overflow in the Sterling area normally occurs from March through September, with the most frequent flooding occurring during June. Annual peak discharges, however, have been recorded throughout the year. Floods in the Sterling area normally occur under nonice-affected conditions. Although their historical frequen~y of occurrence is rare, ice jams are possible on the South Platte River. Irrigation works also influence flooding in the Sterling area by diverting floodwaters, by levee failure, or by combinations of both. Floodwaters on the South Platte River and Pawnee Creek in the vicini ty of Sterl ing can resut t from intense localized rainfall or general rains, either of which may be augmented by snowmel t. The intense rainstorm floods result from intense rainfall over areas of relatively small areal extent. These storms can produce extremely rapid concentrations of floodw~ters and rapid rises in stream levels. Because of little warning time prior to streams attaining flood stages, this type of storm can produce floods that are extremely hazardous. This type of flood produces high peak discharges on tributaries in the general vicinity of the rainstorm. However, the discharges attenuate rapidly on the South Platte River downstream from the contributing tributaries due to valley storage of the floodwater. Erosive effects regardless of can its be considerable in the event origin. Floods resulting of any flood from intense 4