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Douglas County is in the South Platte River watershed. The study areas of the South Platte River, Horse Creek at Deckers, and West Creek are within Pike National Forest. All the other study areas are part of two subbasins, Plum Creek and Cherry Creek. East Plum Creek originates in Pike National Forest and joins West Plum Creek near Sedalia. Plum Creek flows northerly and joins the South Platte River at Chatfield Lake. The climate of the plains area of the county is high inland continental, modified by the Rocky Mountains immediately to the west and Palmer Lake Divide to the south. Precipitation is light with an annual average of 15 to 18 inches. Totals vary substantially for individual years because a large part of the yearly total is from summer thunderstorms. Temperatures in the area range from a high slightly more than 100 degrees Fahrenheit (OF) to a low of approximately -350F. The mean annual air temperature is 470F, and the mean summer air temperature is 660F. The northern one half of the plains area of the county consists of three basic soil types. Loamy and sandy soils exist on the floodplains and terraces. Terrain in these areas is nearly level to gently sloping to moderately steep, with sandy and gravelly soils on the uplands. Vegetation is mainly grass, with some Gambel oak, mountain mahogany, and ponderosa pine trees (U.S. Department of Agriculture, 1971). Most of the remaining area is gently sloping to moderately steep, with loamy soils on the uplands. The southern portion of the plains area is gently sloping to the steep, with sandy soils on the uplands and loamy soils on the tablelands. Vegetation on the uplands is mainly mid-size and talk grasses, with Gambel oak and ponderosa pine trees. The tablelands vegetation is mainly western wheatgrass, junigrass, mountain meehley, and Gambel oak. 2.3 Principal Flood Problems Douglas County is located in an area that is prone to very intense rainfall, sometimes of cloudburst magnitude. Floods have resulted from storms covering large areas with heavy general rainfall as well as from storms covering small area with extremely intense rainfall. Floods generally occur from May through August. The upland areas are characterized by dissected topographic relief with steep stream slopes. Rapid rises, high maximum discharges, short durations, and comparatively low volumes of total runoff characterize the floods. The roadways that cross the streams and obstruct flood flows are the most significant factor affecting flooding in the area. Other manmade objects, such as building, cars, and fences, as well as the natural vegetation of the flood plains, cause flow obstruction. The following accounts of flooding on the South Platte River, Plum Creek, and Cherry Creek area are representative of typical floods for which information is available. 13