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<br />;j::o. <br />o <br />CJ1 <br />14~ <br /> <br />operated from the lower gates since 1954. <br /> <br /> <br />COLORADO RIVER SYSTEM <br /> <br />LAKE POWELL <br /> <br />""" <br />Mudd, River <br />Rl..r <br /> <br />San JUlin <br />, <br />lver <br /> <br />L.. <br />V.n" <br />Wlllh <br /> <br />Figure 1. Map of the Colorado River System (Lake. Mead and <br />Lake Powell). <br /> <br />.The Colorado River inflow to Lake Mead was unregulated <br />prior to 1963 when Glen Canyon Dam was constructed 280 miles <br />{451 km) upstream (Figure 1). Annual discharges are high <br />[8,354,000 acre-feet (10.3 x 109m3) in 1978J, and seasonal <br />discharge peaks usually occur during winter and summer. Dis- <br />charges from Glen Canyon Dam are withdrawn from the hypolim- <br />nion (230 ft, 70 m) of Lake Powell and temperatures range <br />from 7.5-13.50C. The Colorado River inflow, via discharges <br />from Lake Powell, comprises 98% of the inflow to Lake Mead. <br />The remainder is derived from the Virgin and Muddy Rivers, <br />which discharge into the Overton Arm, and Las Vegas \vash, <br />which discharges secondary-treated sewage effluents into Las <br />Vegas Bay (Figure 1). <br /> <br />DATA SOURCES AND METHODS <br /> <br />Historical evaporation data for Lake Mead and discharge <br />. data' for Grand Canyon were obtained from "Surface j.Taters of <br />the United States," U.S. Geological Survey Water-Supply Pa- <br />pers, Part 2L Colora~River Basin, until 1967. Grand Canyon <br />temperature data and salinity data for Lake Mead were de- <br />rived from the "Quality of Surface Waters in the United <br /> <br />3 <br /> <br />~ <br />