Lake Mathews, Water reservoir in Riverside County, California, United States

Lake Mathews is a reservoir in Cajalco Canyon, in the foothills of the Temescal Mountains in Riverside County, California. It was formed by an earthfill dam across the canyon's northern side, with two smaller dikes added in 1961. The reservoir holds 182,000 acre-feet and is the western terminus of the Colorado River Aqueduct.

Construction of the dam began in 1933 and was completed in 1939. Water deliveries through the Colorado River Aqueduct started in 1941, supplying cities across the region through the Metropolitan Water District of Southern California.

The name honors W.B. Mathews, an attorney who helped shape water law and supported the formation of the regional water district. The surrounding area remains mostly closed to the public, allowing the land to function as a protected zone for rare wildlife.

The site is not open to the public, as it serves as the endpoint of a long-distance water supply system and as a wildlife preserve. Visitors can only view the reservoir from outside, such as from public roads in the surrounding area.

The adjacent Lake Mathews Estelle Mountain Reserve covers around 13,000 acres and protects the Stephens's kangaroo rat, a rare rodent. In winter, the area serves as a retreat for eagles that hunt along the water.