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<br />,..,..1t~~'r <br /> <br />d. Ra::'nfall . A low press ure cell, with its assoc ia ted <br />frontal system and a closed circulation aloft, moved slowly <br />across Colorado fran west to east during the period of 5-6 May <br />1973. Shower activity began over the South Platte River basin <br />in Colorado during the evening of 5 May, turned into steady rain <br />at the lower elevations and snow at the 7000-foot level and <br />higher early on 6 May, and ended during the evening of 6 May. <br />Rainfall and snow accumulations were heavy east of the Continental <br />Divide. Rainfall amounts of 6 inches or more were reported in <br />the Kiowa Creek basin; rainfall amounts of 4 inches or more were <br />reported in the Plum, Cherry, Boxelder, Bijou, Badger, and <br />Beaver Creek basins; and 31 inches of new snow were reported at <br />Squaw Mountain. The isohyetal Jll!.ttern for the storm, derived <br />using data fran regular National Weather Service precipitation <br />stations for this area, is shown on plate 3. Since a rainfall <br />"bucket survey" was not conducted for this storm, the isohyetal <br />pattern should be considered approximate. Mass rainfall curves <br />at selected locations are shown on plate 4. The 24-11our storm <br />depth-area amounts of 5-6 May are shown on plate 5. <br /> <br />12. FLOOD HYDROLOGY - SOUTH PLATTE RIVER BASIN <br /> <br />a. General. Snowmelt runoff from the lower mountain area <br />of the South Platte River basin began about the middle of April. <br />By the first of May, flow had increased to about 500 cubic feet <br />per second at Waterton, Colorado; about 1,000 cubic feet per <br />second in the Denver area, and about 2,000 cubic feet per second <br />in the reach from Kersey to the State line. Rainfall runoff, <br />which ..as the major causative factor of the flooding in the South <br />Platte River basin, began on 5 May. Sharp increases in flow as <br />a result of the rainfall runoff were recorded at all gaging <br />stations along the South Platte River from Littleton to the <br />Colorado-Nebraska State line. The rainfall runoff was augmented <br />by mountain snowmelt runoff which 'W8.S also increasing during <br />this period. The result was general flooding throughout the <br />South Platte River basin; flooding was characterized by high, <br />sharp hydrograph peaks from the rainfall runoff followed by a <br />slow recession because of the continuing mountain snowmelt runoff. <br />Bankfull discharges were experienced along portions of the main <br />stem of the South Platte River for most of the month of May and <br />on into June. <br /> <br />g. Peak discharp:es. Peak discharges experienced at the <br />South Platte River gaging stations are sUlblllarized in table 3. <br />As indicated from these data, the Kersey and Weldona stations <br /> <br />18 <br />