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<br />The climate of the plains area of Douglas County is high, inland <br />continental, modified by the Rocky Mountains immediately to the <br />west and Palmer Lake Divide to the south. Precipitation is light, <br />with an annual average of 15 to 18 inches. Totals vary <br />substantially for individual years because a large part of the <br />yearly total is from summer thunderstorms. Temperatures in the <br />area range from a high of slightly more than 1000F to lows of <br />approximately -350F. The mean annual temperature is 470F. <br /> <br />The northern one-half of the plains area of Douglas County con- <br />sists of three basic soil types. Loamy and sandy soils exist on <br />the flood plains and terraces. The terrain in these areas is <br />nearly level to gently sloping to moderately steep, with sandy and <br />gravelly ~oils on the uplands. Vegetation is mainly grass, with <br />some Gambel oak, mountainmahogany, and ponderosa pine trees <br />(Reference 2). Most of the remaining area is gently sloping to <br />moderately steep, with loamy soils on the uplands. <br /> <br />2.3 Principal Flood Problems <br /> <br />Douglas County is located in an area that is subject to intense <br />rainfall, sometimes as cloudbursts. Floods have resulted from <br />storms covering large areas with heavy general rainfall as well as <br />from storms covering small areas with intense rainfall. Floods <br />generally occur from May through August. The upland areas are <br />characterized by dissected topographic relief and steep stream <br />slopes. Floods are characterized by rapid rises, high maximum <br />.discharges, short durations, and comparatively low volumes of <br />total runoff. <br /> <br />The roadways that cross streams and obstruct floodflows are the <br />most significant factor affecting flooding in the area. Other <br />manmade features, such as buildings, cars, and fences,.as well as <br />the natural vegetation of the flood plains, cause flow <br />obstruction. <br /> <br />The following accounts of flooding on Cherry Creek in the study <br />area are representative of typical floods for which information is <br />available. <br /> <br />Three floods occurred during May and June 1864. The first, <br />originating in the Cherry Creek and Plum Creek basins, occurred <br />during the nights of May 19-20, 1864, and was caused primarily by <br />a cloudburst in the upper part of those basins. On the morning of <br />May 20, the flood covered the lower portions of Denver with water <br />to depths of from 1 to 5 feet, leaving great deposits of sand and <br />gravel. <br /> <br />Records indicate that 2.08 inches of rain occurred during a 2-hour <br />period on July 14, 1912. The heaviest precipitation occurred <br /> <br />5 <br />