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Half a dozen little mountain brooks flow into Mono Lake, but not a stream of any kind flows out of it. What it does with its surplus water is a dark and bloody mystery.

– Mark Twain

Mono Lake lies on the eastern slope of the Sierra Nevada near the east entrance to Yosemite National Park. Mono Lake is a terminal lake, which means that although water enters the lake by precipitation and by streams and ground water flowing into it, water can leave only by evaporation. The lake level fluctuates with climatic changes. The volume of water that leaves the lake by evaporation is the product of the surface area times the depth of evaporation. If the volume that leaves by evaporation is exactly balanced by the inflow, the lake level will not change. If the inflow exceeds evaporation, the water level will rise. If the inflow is less than evaporation, the lake level will fall. The Mono Lake basin has an area of 695 square miles (mi2) [180,000 hectares (ha)]. Inputs to the lake under natural conditions are direct precipitation, with an estimated annual average of 8 in. (0.2 m); runoff from the land areas via gauged streams, which is estimated to average 150,000 acre-feet (ac-ft)* per year [1.85 x 108 cubic meters (m3)]; and ungauged runoff and groundwater inflow, which is estimated to average 37,000 ac-ft per year (4.56 x 107 m3). The average annual rate of lake evaporation is about 45 in. (1.1 m) (Vorster 1985). When it was first surveyed in 1856, the elevation of Mono Lake was 6407 ft (1953 m) above sea level. Climatic effects of moister and drier periods caused the lake level to rise to as much as 6428 ft (1959 m) in 1919 and then to fall to 6410 ft (1954 m) by 1941. In that year, water was first diverted from four of the five major streams feeding Mono Lake into the Los Angeles Aqueduct and then into southern California.

* An acre-foot is a measure of the volume of water that is commonly used in the western United States. It is the amount of water that will cover an acre of land to a depth of 1 foot (43,560 ft3 or 1234 m3)

Since the beginning of diversions in 1941, the surface elevation of Mono Lake has declined substantially (Figure 1.6). Diversions amounted to as much as 100,000 ac-ft (1.23 x 10 8m3) per year. The historic low was reached in December 1981, when the lake elevation was 6372.0 ft (1942.2 m). The decline was arrested and the level rose to 6381 ft (1945 m) during a very wet period from 1982 through 1984. A return to more normal precipitation conditions meant that the lake level began to fall again. In 1989, the diversions were halted under a temporary court restraining order that prohibited any such diversions that would result in a lake level of less than 6377 ft (1944 m). However, even without any diversions the level of Mono Lake still declined due to very dry conditions in the eastern Sierra Nevada, so that by the end of 1992 it was 6373.5 ft (1942.6 m).

In 1994 the California State Water Resources Control Board issued Decision 1631 which established permanent stream-flow values for the tributary streams to Mono Lake in order to protect fish in the streams. In addition, a permanent lake elevation of 6,392 ft (1949 m) was set for Mono Lake. No diversions of influent water are permitted when the lake level is below that set by the decision.

In 1941, the year that diversions began, the surface area of Mono Lake was 53,500 ac (21,670 ha). When the lake elevation declined by 38 ft (12 m) from 1941 to 1981, the surface area shrank to about 40,000 ac (16,200 ha). The annual diversion of 100,000 ac-ft (1.23 x 108 m3 ) would cover the 40,000 ac (16,200 ha) lake to a depth of 2.5 ft (0.76 m). The water level fell because the amount of the diversion plus the natural evaporation from the lake was far in excess of the amount of precipitation onto the lake surface plus the remaining surface inflow and the groundwater inflow.

Since 1992, sufficient precipitation has fallen over the Mono Lake drainage basin for lake levels to rise inasmuch as no diversions were permitted. As of January 2000 the lake level stood at 6384.3 ft (1947.2 m). It will take several more years before the lake level reaches the minimum elevation of 6392 ft (1949 m) set in Decision 1631.

One consequence of the volume reduction of Mono Lake was an increase in the salinity of the lake. In its original, natural condition with a surface elevation of 6410 ft (1954 m), the salinity of Mono Lake was 5.4%. At an elevation of 6377 ft (1944 m), the salinity rose to 9.3%. The increase in salt concentration resulted in a reduction of the brine shrimp population of the lake. There is a commercial fishery in Mono Lake for brine shrimp, and the shrimp also serve as an important food source for nesting and migratory birds. Brine flies also inhabit the shallow waters of the lake edge and provide a second food source for the many species of birds that migrate through the area and the nesting colonies of California gulls and snowy plovers. With the elimination of diversions from the lake, and the subsequent rise in water levels, the salinity has been reduced to 8.1%.

Material from Fetter, Applied Hydrogeology, 4th Edition, Prentice-Hall, 2001, and the Mono Lake web site, http://www.monolake.org