The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough that stretches to the Gulf of California in Mexico. Over millions of years, the Colorado River has flowed into the Imperial Valley and deposited alluvium (soil), creating fertile farmland, building up the terrain, and constantly moving its main course and river delta. For thousands of years, the river has alternately flowed into the valley, or diverted around it, creating either a saline lake called Lake Cahuilla, or a dry desert basin, respectively. When the Colorado River flows into the valley, the lake level depends on river flows and the balance between inflow and evaporative loss. When the river diverts around the valley, the lake dries completely, as it did around 1580. Hundreds of archaeological sites have been found, indicating possibly long-term Native American villages and temporary camps.
The current lake was formed from an inflow of water from the Colorado River in 1905. Beginning in 1900, an irrigation canal was dug from the Colorado River to the old Alamo River channel to provide water to the Imperial Valley for farming. The headgates and canals sustained a buildup of silt, so a series of cuts were made in the bank of the Colorado River to further increase the water flow. Water from spring floods broke through a canal head-gate diverting a portion of the river flow into the Salton Basin for two years before repairs were completed. The water in the formerly dry lake bed created the modern lake, which is about 15 by 35 miles (24 by 56 km).
The lake would have dried up, but farmers used generous amounts of Colorado River water and let the excess flow into the lake. In the 1950s and into the '60s, the area became a resort destination, and communities grew with hotels and vacation homes. Birdwatching was also popular as the wetlands were a major resting stop on the Pacific Flyway.
In the 1970s, scientists issued warnings that the lake would continue to shrink and become more inhospitable to wildlife. In the 1980s, contamination from farm runoff promoted the outbreak and spread of diseases. Massive die-offs of the avian populations have occurred, especially after the loss of several species of fish on which they depend. Salinity rose so high that large fish kills occurred, often blighting the beaches of the sea with their carcasses. Tourism was drastically reduced.
After 1999, the lake began to shrink as local agriculture used the water more efficiently so less runoff flowed into the lake. As the lake bed became exposed, the winds sent clouds of toxic dust into nearby communities. Smaller amounts of dust reached into the Los Angeles area and people there could sometimes smell an odor coming from the lake. The state is mainly responsible for fixing the problems, and California lawmakers pledged to fund air-quality management projects in conjunction with the signing of the 2003 agreement to send more water to coastal cities. Local, state, and federal bodies all had found minimal success dealing with the dust, dying wildlife, and other problems for which warnings had been issued decades before. At the beginning of 2018 local agencies declared an emergency and, along with the state, funded and developed the Salton Sea Management Program. After a slow start and some small projects, construction started on a $206.5 million project in early 2021 on the delta of the New River, creating ponds and wetlands on the southern shore of the lake.
In 2020, Palm Springs Life magazine summarized the ecological situation as "Salton Sea derives its fame as the biggest environmental disaster in California history".
In the 1920s, agriculture had boomed in the valley as the Imperial Irrigation District delivered large quantities of Colorado River water to irrigate the crops. The lake would have dried up naturally, but with flood irrigation being commonly used, plenty of water ran off the farms into the lake and kept it full. The district holds senior rights to water from the Colorado River according to Doctrine of Prior Appropriation, which is based on whoever first put the water to beneficial use could continue to claim it. In 1930, a wildlife refuge was established on some wetlands along the edge of the lake that had attracted many birds. The fish flourished in the lake and provided a source of food for massive populations of migratory birds. Birdwatchers flocked to this new refuge in the middle of a desert.
The continuing intermittent flooding of the Imperial Valley from the Colorado River ended with the construction of Hoover Dam. The Imperial Dam, built in 1938, serves as a desilting dam for water entering the irrigation canals.
In the 1950s and into the 1960s, the communities expanded as the area's reputation as a resort destination and sport fishery grew. Hotels and yacht clubs were built on the shore along with homes and schools. Resorts in communities like Bombay Beach hosted entertainers such as Frank Sinatra, The Beach Boys and Bing Crosby. Yacht clubs held parties at night and golf courses provided recreation. Many people came for boating activities such as water skiing and fishing as stocked fish proliferated. Lakeshore communities grew as vacation homes were built. More than 1.5 million visitors visited annually at the peak.