The Salton Sea is a shallow, landlocked body of water that has a high concentration of salts in Riverside and Imperial counties, on the San Andreas Fault at the southern end of the U.S. state of California. It lies 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 (or only) river delta. For thousands of years, the river has alternately flowed into and out of the valley, creating a freshwater lake or an increasingly saline lake, and a dry desert basin, respectively, depending on river flows and the balance between inflow and evaporative loss. Hundreds of archaeological sites have been found, indicating possibly long-term Native American villages and temporary camps.