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0+::- o en rv Dam. 'Phe Hoover Dam intake towers, being located at the dis- tant end of the reservoir, would be one of the last areas in Lake Mead to be influenced by cold-water discharges. Temper- atures at the dam could be considerably higher than else- where in the reservoir, particularly in comparison to the Upper Basin. Estimates of reservoir-wide evaporation based on data from the Hoover Dam intake towers could, therefore, also be higher than actual evaporation. Hydrologists at the Bureau of Reclamation have consistently observed an overall gain of water in Lake Mead. Based on a ten-year average dur- ing 1960-1970, the measured Lake Mead contents exceeded wa- ter budget estimates by approximately 230,000 acre-feet (2.84 x 108m3)/yr (USBR data). This could, in part, be due to an overestimate of evaporation from the reservoir since 1963 when advection was altered by construction of Glen Canyon Dam. , Although measured evaporation rates may be somewhat too high in the period after 1963, it is still evident that cold-water discharges from Glen Canyon Dam have significant- ly reduced evaporation from Lake 1,lead. If we exclude the 1975 and 1976 values, which are clearly too high, pre- and post-Lake Powell evaporation rates average 85.2 inches (216 cm)/yr and 76.8 inches (195 cm)/yr. This is equivalent to a reduction in annual water loss of at least 93,376 acre-feet (1.2 x 108m3), which is very similar to predictions made during the mid-1960s. Government scientists reported that cold-water discharges would reduce evaporation by about 100,000 acre-feet (1.23 x 108m3)/yr. Operation of Glen Can- yon Dam from a deep discharge is thus an extremely effective method of reducing evaporation from Lake Mead. Manipulation of Evaporation Rates It has long been known that reservoirs operated from a deep discharge store heat, whereas, those operated from a surface discharge dissipate heat [9J. The principle here is quite simple and depends only on the formation of thermal gradients in the reservoir. In Lake Mead, surface tempera- tures exceed hypolimnion temperatures during all periods of the year, except winter ,<hen the reservoir is completely mixed and isothermal. The temperature gradient is particu- larly sharp during summer when surface temperatures reach 27-30oC, compared to 11-120C in the hypolimnion. In the period from October, 1977 - September, 1978, operation of Hoover Dam from the deep discharge resulted in an average, net advective heat gain of 9.04 cal/cm2.day (Table II). However, this would have decreased to -29.55 cal/cm2'day if the dam had been operated from a surface discharge over this period. The net difference in advection between surface and deep discharge would be -38.59 cal/cm2'day (Table II). Using 11