The Salton Sea, California's largest lake, is located in the southeastern desert of California. It lies within a 7851 square mile closed basin that includes the Coachella and Imperial valleys of California, and the Mexicali Valley of Mexico with a surface elevation of 227 ft below mean sea level (msl). The shallow nature of the sea, with a surface area of 367 mix2 (951 kmx2) and a depth of 51 ft (15.5 m), renders it very sensitive to even slight changes of inflow. Over 85% of the water entering the Salton Sea results from agricultural run-off, 1.34 million acre ft (Maf), with less than 3% of inflow from annual precipitation. The Salton Sea is situated in the Colorado Desert in one of the most arid regions of the United States. Annual precipitation is less than 3 in. (7.6 cm), and mean monthly temperatures in July are 92°F (33.3°C), with maximum temperatures exceeding 100°F (37.7°C) on more than 110 day yr-1. Potential evaporation is estimated at 5.78 ft (1.76m) per year.
The Salton Basin, once connected to the Gulf of California, is characterized by a shallow marine environment. For the past several million years, sediments that once filled the Grand Canyon were eroded by the Colorado River and deposited in the Gulf of California, eventually building a huge delta, blocking off the Salton Basin from the ocean. Once separated from the Gulf of California, the Salton Basin would periodically dry out as the Colorado River drained directly into the Gulf of California. At other times, the river would change course and fill the basin. These prehistoric inundations have been called Lake Cahuilla or Lake LeConte.
The present-day Salton Sea was formed in 1905 when flood flows on the Colorado River rushed into the Salton Basin (Salton Sink) through a temporary diversion channel until the breach was finally filled by the Southern Pacific Railroad Company in 1907. After an initial high stand of about -195 ft msl, the sea receded to about -250 ft msl by 1920. Since then, with the expansion of agriculture in the Imperial and Coachella valleys and increased agricultural run-off, the surface elevation of the sea has risen to its current elevation of about -227 ft msl. The elevation of the sea has remained relatively stable at its current level since the 1980s, indicating that the inflow of about 1.34 Maf is equal to evaporation at that elevation.
More than 400 species of invertebrates, mostly single celled plankton, have been identified in the sea. These provide the food base supporting a highly productive fishery, with an estimated 200 million fish-one of the most productive fisheries in the world. The Salton Sea is of critical importance for many species of migratory birds. The sea supports over 90% of the North American population of eared grebes, with as many as three million individuals during migration, as many as 30,000 American white pelicans and 2000 brown pelicans, more than 120,000 shorebirds of 44 species, 25,000 snow and Ross' geese, the largest breeding colony of gull-billed terns in Western North America, and 45% of endangered Yuma clapper rail habitat.
Approximately 4.5 million tons of salts are added to the sea annually. Because evaporation is the only outlet from the sea, the dissolved salts, nutrients, and minerals that enter the sea remain there, and have accumulated over the past century to the point at which the sea is now about 25% saltier than the ocean, at about 44,000 mg L. Increasing salinity may cause the fishery to collapse as it approaches 60,000 mg L. At the present rate of salt loading of about 4.5 million tons per year, the Salton Sea would reach the 60,000 mg L threshold in about 50 yr, assuming inflow remains at its present level of 1.34 Maf per year.
Nutrient loading from agricultural run-off has created eutrophic conditions at the Salton Sea. Productivity and biomass are very high, leading to oxygen depletion caused by decay of accumulated biological material. These anoxic conditions have contributed to extensive fish kills over the past few decades, leading to further oxygen depletion. Eutrophic conditions worsen during summer months, when high biological productivity and warm water temperatures conspire to greatly reduce dissolved oxygen throughout the water column. One event, in August 1999, resulted in the death of six to seven million fish over a period of several days. Chemical data taken at the time indicated a complete loss of dissolved oxygen in portions of the sea coincident with that event.
With reduced inflows from water transfers, conservation measures and increased demand, salinity increases rapidly and the contracting lake concentrates salts already in residence while more salts continue to enter the sea in agricultural run-off. With a reduction of inflow by 300,000 af per year, salinity would reach the 60,000 mg L threshold for the fishery in about 12 yr; and with a reduction of 500,000 af, salinity would reach the limit of tolerance of the fishery in just 7 years. The collapse of the fishery would represent a serious adverse environmental impact. The death of 200 million fish would have a cascade effect on the rest of the ecosystem, causing the demise of fish-eating bird populations, exacerbating eutrophication from decomposition of the dead fish, and creating a huge breeding ground for flies and other pathogens in their rotting carcasses. Many species of birds would be critically impacted. Ground-nesting bird colonies on Mullet Island-the only island in the sea-would be exposed to coyotes, cats, and other predators with a draw down of only 7 feet. Many other species would experience substantial whole-species population declines. Other potential impacts of reduced inflows to the Salton Sea include collapse of lake-related economies, such as boating, hunting, fishing, and property values; degradation of air quality as a result of exposure of as much as 120 mix2 of fine lake bottom sediments to the desert winds; loss of agricultural productivity from salt and dust deposition; and increased respiratory disease and human health problems as a result of airborne sediments.
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