Use of hydroelectric dams to control evaporation and salinity in the Colorado River system

The main stem reservoirs on the Colorado River comprise one of the largest and most heavily used freshwater bodies in the nation. These reservoirs (Lake Powell, Lake Mead, Lake Mohave and Lake Havasu) can store up to 53,590,400 acre-feet of water at their maximum capacities. Nonetheless, local water shortages still exist in some areas of the Colorado River Basin. There is also concern that salt concentrations are approaching levels that could severely affect municipal and agricultural uses. Water shortages will become even more acute as demands for water increase with continued urban and agricultural development in the basin. Water conservation and salinity control programs have already been adopted, or are under investigation, in most states using Colorado River water. Reductions in consumptive water uses through more efficient irrigation practices, power plant cooling and wastewater reuse will, to some extent, help alleviate future water shortages. However, this will not offset the rising demands, and basin-wide shortages could occur by the year 2000. Similarly, recent estimates indicate that salt concentrations in the river at Imperial Dam will rise to 1150 mg/1 as a result of flow depletions projected to occur during this century M,3j. Construction of salinity control projects approved by Congress under PL 93-320 will significantly reduce salinity, but implementation of these projects will be costly and time consuming. Water shortages and salinity control in the Colorado River system have thus far been addressed from the standpoint of reducing water uses and controlling point source salt inputs. Little attention has been given to investigating methods of reducing evaporation from the reservoirs, but studies conducted in 1952 and 1953 showed that it was a major water loss from the Colorado River system. Moreover, 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 Republic, May 19, 1966; Phoenix Gazette, July 28, 1966). Our analysis of historical evaporation data, and recent investigations in Lake Mead 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 influence on the heat budget of Lake Mead. Historically, the Colorado River inflow contributed large quantities of heat to the reservoir during the spring and early summer. However, the construction of Glen Canyon Dam and formation of Lake Powell in 1963 altered the natural temperature and flow cycles of the river. Discharges of cold water from the hypolimnion (230 ft, 70 m) of Lake Powell have significantly reduced energy inputs to Lake Mead. Similarly, it appears that heat losses from the reservoir could be increased 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/yr and result in considerable 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 minimize evaporative water losses from Lake Mead and reduce salinity in the Colorado River