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. . . STUDY AREA DESCRIPTION Area Description Elbert County is southeast of Denver, Colorado, within one of Colorado's cloudburst zones about 50 miles east of the Front Range foothills (See Appendix Figure 1). The rolling high plains of the area contain many creek channels which are dry much of the time, yet are in fact wide streambeds carved by floodwaters. This seems unusual, considering that precipitation in the area averages but 12 to 16 inches per year. However, two-thirds of that amount usually occurs as intense flood-producing thunderstorms during the summer months. Most of the streams crossing Elbert County begin near its southwestern corner, where a narrow plateau at an altitude of 7,500 feet forms the divide between the South Platte River and the Arkansas River basins. As the streams traverse Elbert County in a northerly or easterly direction, their slopes decrease from more than 100 feet per mile in the headwaters to less than 50 feet per mile at the Arapahoe-Elbert County line downstream, where altitudes average about 5,500 feet. Drainage areas of the creeks within the county range from about 30 to 300 square miles. Floodinq Problems and Flood History Major flooding occurred in Elbert County during 1878, 1935, and 1965. Although the most extreme peak-discharge values were recorded during the widespread floods of 1935 and 1965, localized intense storms covering a few square miles occur randomly throughout Elbert County every year. The most recent flooding occurred on July 17, 2000 when a severe storm dumped heavy rainfall on portions of western Elbert County. Total storm precipitation of up to 4-6" of rain in about one hour was reported by at least one landowner. Several left-bank tributaries to Running Creek (Boxelder Creek) were affected by the storm. County Road 13 was washed out, and several homes suffered damage from floodwaters. Discharge estimates ranged from 361 cfs/square mile to 897 cfs/square mile. See the CWCB Field Report, "Elbert County Flood of July 17,2000" for additional information. Tributary streams occasionally may produce flood discharges approaching several times that shown by a 100-year curve. However, because the storms are generally localized, the volume of runoff is small and the main channel downstream usually can pass the additional water at heights below the 1 DO-year flood levels. In this case, major damage is confined to the immediate area affected by the cloudburst. Water from these intense storms, which usually occur during May through August, may flow in sheets down the steep hillsides until it reaches an existing waterway. Material gouged from the flooded area is carried downstream. As the flood wave slows, the eroded sediment is deposited onto the flood plain of the receiving stream. Areas once modified by floods, as evidenced by the wide stream valleys in Elbert County, probably will be covered by floodwaters again. The June 1965 flood occurred after the floodwater-retarding structures of the Kiowa Watershed Project had been built by the U.S. Soil Conservation Service (SCS). These SCS floodwater-retarding 2