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~ o CJl c.u high evaporation directly influences salinity because it increases the concentration of salts in the reservoirs. Although various schemes have been offered for reducing evaporation from Lake Mead, it has usually been viewed as an uncontrollable water loss. However, during the mid-1960s, U.S. Geological Survey and Bureau of Reclamation scientists estimated that cold-water discharges from Glen Canyon Dam would reduce evaporation in Lake Mead. The estimates were never published in report form but did appear in internal government memoranda and newspaper articles (Arizona Repub- lic, May 19, 1966; Phoenix Gazette, July 28, 1966). Our analysis of historical evaporation data, and recent inves- tigations in Lake Mead [5J indicate that evaporation did indeed decrease after Lake Powell was formed in 1963. Advective energy (heat) inputs (Colorado River inflow) and outputs (Hoover Dam discharge) have a significant influ- ence on the heat budget of Lake Mead [4,6J. Historically, the Colorado River inflow contributed large quanti ties of heat to the reservoir during the spring and early summer, However, the construction of Glen Canyon Dam and fOl~ation of Lake Powell in 1963 altered the natural temperature and flow cycles of the river [7J. Discharges of cold water from the hypolimnion (230 ft, 70 m) of Lake Powell have signifi- cantly reduced energy inputs to Lake Mead. Similarly, it appears that heat losses from the reservoir could be in- creased if Hoover Dam were operated from a surface, rather than deep-water, discharge. The combined effects of a cold- water discharge from Glen Canyon Dam and a surface discharge from Hoover Dam could reduce evaporation from Lake Mead by over 200,000 acre-feet (2.47 x 108m3)/yr and result in con- siderable decreases in salinity. The purpose of this paper is to present data in support of these conclusions and to describe how the hydroelectric dams can be operated to. min- imize evaporative water losses from Lake Mead and reduce salinity in the Colorado River. STUDY AREA Lake Mead was formed in 1935 by construction of Hoover Dam. It extends 114 miles (183 km) from the mouth of Grand Canyon to Black Canyon, the site of Hoover Dam (Figure 1). Lake Mead is one of the largest reservoirs' in the country with a surface area of 163,088 acres (660 km2) and a volume of 29,185,245 acre-feet (36 x 109m3), at the maximum operat- ing level of 1227 ft (374 m) [8J. It is separated into two large basins by Boulder Canyon, located midway through the reservoir (Figure 1). The area above Boulder Canyon is re- ferred to as the Upper Basin and that below as the Lower Basin. Hoover Dam is equipped with intake gates at 1045 ft (319 m) and 895 ft (273 m) elevations. The dam has been 2