The Salton Sea is a shallow, landlocked, highly saline endorheic lake in Riverside and Imperial counties in Southern California. It lies on the San Andreas Fault within the Salton Trough, which stretches to the Gulf of California in Mexico. The lake is about 15 by 35 miles (24 by 56 km) at its widest and longest. A 2023 report put the surface area at 318 square miles (823.6 km²). The Salton Sea became a resort destination in the 20th century, but saw die-offs of fish and birds in the 1980s due to contamination from farm runoff, and clouds of toxic dust in the current century as evaporation exposed parts of the lake bed.

Over millions of years, the Colorado River had 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 alternately flowed into the valley or diverted around it, creating either a salt lake called Lake Cahuilla or a dry desert basin, respectively. When the river diverted around the valley, the lake dried completely, as it did around 1580. Hundreds of archaeological sites have been found in this region, indicating possibly long-term Native American villages and temporary camps.

The modern 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 provide water to the Imperial Valley for farming. 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.

During the early 20th century, the lake would have dried up, except that farmers used generous amounts of Colorado River water for irrigation and let the excess flow into the lake. In the 1950s and into the 1960s, 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 wildlife 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. The state is mainly responsible for fixing the problems. 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. In 2017, the Salton Sea Management Program was developed by the state. The Torres Martinez Desert Cahuilla Indians partnered with the state to restore shallow wetlands along the northern edge of the sea in 2018. Construction began in 2021 on the 4,110-acre (1,660 ha) Species Conservation Habitat (SCH) restoration and dust suppression project on the small delta of the New River. In 2025, water began flowing into the first 2,000 acres (810 ha) of the SCH complex of shallow ponds.

The Gulf of California would extend as far north as the city of Indio, some 150 miles (240 km) northwest of its current limits, were it not for the delta created by the Colorado River. Over three million years, through all of the Pleistocene, the river's delta expanded until it cut off the northern part of the gulf. Since then, the Colorado River has alternated between emptying into the basin, creating a freshwater lake, and emptying into the gulf, leaving the lake to dry and turn to desert. Wave-cut shorelines at various elevations record a repeated cycle of filling and drying over hundreds of thousands of years. The most recent freshwater lake was Lake Cahuilla, also known as the Blake Sea after American professor and geologist William Phipps Blake. It covered over 2,000 square miles (5,200 km²), six times the area of the Salton Sea.

Archaeological sites and radiocarbon dates indicate that the lake was filled three or four times over the last 1,300 years. When full, the lake would attract Native Americans to its shores. Hundreds of sites have been found, some possibly long-term villages and others temporary camps. The occupants ate at least four species of fish (two of which were razorback sucker and bonytail chub), birds (particularly the coot), black-tailed jackrabbit, black-tailed cottontail rabbit, and sometimes deer and bighorn sheep. Among the plants they used were bulrush, cattail, mesquite, and saltbush. The Cahuilla people have an oral memory of the last lake, which existed in the 17th century and dried up soon after 1700.

Throughout the Spanish period of California's history, the area was referred to as the "Colorado Desert" after the Colorado River. In a railroad survey completed in 1855, it was called "the Valley of the Ancient Lake". On several old maps from the Library of Congress, it has been found labeled "Cahuilla Valley" (after the local Indian tribe) and "Cabazon Valley" (after a local Indian chief – Chief Cabazon). "Salt Creek" first appeared on a map in 1867, and "Salton Station" is on a railroad map from 1900, although this place had existed as a rail stop since the late 1870s. Until the advent of the modern sea, the Salton Sink was the site of a major salt-mining operation.