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and is operated by the U.S. I\.rmy Corps of Engineers to provide additional flood control for part of the Denver metropolitan area. Between the gaging stations at Denver and Henderson, the South Platte River receives the discharges of Clear Creek and the Denver municipal wastewater treatment plant. Flows in this reach also are affected by irrigation diversions and return flows. Mean annual flow at Henderson was 266,000 acre-feet during the 1928 through 1979 water year period (Table 8). Clear Creek - This stream originates in the higher elevations along the eastern slope of the Continental Divide. The basin covers an area of about 580 square miles. It can be characterized as a mountain tributary, with about 75 percent of its runoff resulting from the melting snowpack in the upper reaches of the sub-basin during May through July. Mean annual runoff for the 1928 through 1979 water year period was nearly 66,000 acre-feet at the gaging station on Clear Creek near its confluence with the SOuth Platte River (Table 8). By the time it reaches the gaging station at Kersey, the SOuth Platte River has gained streamflows of St. Vrain Creek, the Big Thompson River and the Cache la poudre River. These streams, which originate in the mountains to the west, contribute substantial quantities of flow from both native surface waters and transbasin imports, as discussed below. St. Vrain Creek - The dr~inage area of St. Vrain Creek near its confluence with the SOuth Platte River at Platteville, Colorado is about 980 square miles. This includes the drainage of Boulder Creek, a major tributary in this sub-basin. The upper 30 percent of this sub-basin is mountainous. Mean annual runoff for St. Vrain Creek near its mouth was about 146,000 acre-feet for the 1928 through 1979 water year period. -30- I I I I I I I I I I I I I I I I I I I