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<br />DISCUSSION <br /> <br />Area Description <br /> <br /> <br />Elbert County is southeast of Denver, Colo.. within one of Colo- <br />rado's cloudburst zones about 50 miles east of the Front Range foot- <br />hills. The'rolling h~gh plains of the area contain many creek channels <br />which are dry much of the time, yet are in wide streambeds carved by <br />floodwaters. This seems unusual. considering that precipitation in the <br />~rea averages but 12 to 16 'inches per year. However. two-thirds of <br />that amount usually occurs as Intense flood-producing thunderstorms <br />during the summer ,months. <br /> <br />Most of the streams crossing Elbert County begin near its south- <br />western corner, where a narrow plateau at an altitude of 7.500 feet <br />forms the divide between the South Platte River and the Arkansas River <br />basins. As the streams traverse Elbert County in a northerly or east- <br />erly direction, their slopes decrease from more than 100 feet per mile <br />in the headwaters to less than 50 feet per mile at the Arapahoe-Elbert <br />County line downstream, where altitudes average about 5.500 feet. <br />Drainage areas of the creeks within the county range from about 30 to <br />300 square miles. <br /> <br />Flood History <br /> <br />Major flooding occurred in Elbert County during 1878, 1935. and <br />1965. Although the most extreme peak-discharge values were recorded <br />during the widespread floods of 1935 and 1965. localized intense <br />storms covering a few square miles occur 'randomly throughout Elbert <br />County every year. Thus tributary streams. occasionally may produce <br />flood discharges approaching several times that shown by a 100-year <br />curve. However. because the storms are generally localized, the volume <br />of runoff is small and the main channel downstream usually can pass <br />the additional water at heights below the 100-year flood levels. In <br />this case. major damage is confined to the immediate area affected by <br />the cloudburst. <br /> <br />Water from these intense storms, which usually occur during May <br />through August. may flow in sheets down the steep hillsides until it <br />reaches an existing waterway. Material gouged from the flooded area <br />is carried downstream. As the flood wave slows. the eroded sediment <br />is deposited onto the flood plain of the receiving stream. <br /> <br />Areas once modified by floods, as evidenced by the the wide <br />str~am valleys in Elbert County, probably will be covered by flood- <br />waters again. The June 1965 flood occurred after the floodwater- <br />retarding structures of the Kiowa Watershed Project had been built by <br />the U.S. Soil Conservation Service (SCS). These SCS floodwater- <br />retarding structures, which store about 1 inch of runoff behind their <br />emergency spillways, delay tributary flows to Kiowa Creek. This stor- <br />age decreases natural-flow peak discharges and helps to reduce down- <br />stream property damage. Thus, even though retarding dams are placed <br />across numerous tributaries, unusually large floods still can result <br />from intense thunderstorms. <br /> <br />2 <br />