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STUDY AREA DESCRIPTION Community Description Logan County is located in eastern Colorado within the South Platte River valley, The County is bounded by the Colorado-Nebraska state line on the north, Phillips and Sedgwick Counties on the east, Washington County and Morgan Counties on the south, and Weld County on the west. Sterling is the county seat of Logan County, and is located approximately 110 air miles northeast of Denver, Highway 76 is the major east-west highway that serves the county, The South Platte River bisects the county and generally flows through it from southwest to northeast. Elevations in the county range from about 3,600 feet to over 5,000 feet. The area studied for the FrS is the unincorporated areas of Logan County as of September 29, 1989. Watershed Description The South Platte River flows through Logan County from southwest to northeast and has a generally braided channel with broad, shallow floodplain areas. The average streambed slope through Logan County is less than 6 feet per mile, The drainage area of the South Platte River at Crook is 19,238 square miles. Most of the major tributaries in Logan County are left-bank tributaries that flow into the South Platte River from the plains area to the north, The headwaters of the South Platte River originate at the Continental Divide with elevations in excess of 14,000 feet. The South Platte River, along with the tributaries listed above, are just some of the major flooding sources within the unincorporated areas of the county, ~ Floodinl! Problems and Floodinl! History Flooding on the South Platte River normally occurs during the spring and summer months from May through September as a result of flooding from thunderstorms or general rainstorms which can be enhanced by rapid snowmelt runoff from the upper watershed areas, Flooding on the tributary streams in Logan County are almost always a result of intense thunderstorms or heavy general rainstorms during the late spring and summer months, Information on flooding problems has been partially extracted from the Flood Insurance Study, The Pawnee Creek overflow is created by the V,S, Highway 6 and railroad bridge crossings of Pawnee Creek. During periods of high stage on Pawnee Creek, floodwaters can escape the Pawnee Creek valley to flow overland near Riverside Cemetery toward Sterling, Flood flows on the South Platte River are obstructed by highway and bridge crossings, Intense rainstorms caused by cloudburst events can be extremely hazardous due to the rapid rise of floodwaters from the storm runoff. This type of storm event produces high discharges on tributary streams in the vicinity of the storm, However, the discharges attenuate quite rapidly on the South Platte River downstream from the tributaries due to valley storage and other factors, Other Studies Several floodplain information reports have been produced for areas within Logan County, The following list summarizes other studies that have been published for unincorporated areas of Logan County. '- 2