Lake Mathews is located in the Cajalco Valley in the foothills of the Santa Ana Mountains in Riverside County. It is the western terminus for the Colorado River Aqueduct that provides much of the water used by Metropolitan's member agencies.
Construction on the reservoir began in 1933, at the same time the CRA was being built across the desert. The dam and intake structure were completed in 1939. The first water arrived from the Colorado River in February 1940, and water deliveries began in 1941. In 1961, the reservoir's capacity was nearly doubled to its current capacity of 182,000 acre-feet through the construction of two small dikes.
Lake Mathews is surrounded by approximately 4,000 acres of protected land. In 1982, this land was declared a State ecological reserve. In the early 1990s, an additional 9,000 acres was added to the reserve after the discovery of the endangered Stephens kangaroo rat in the area. The area is now called the Lake Mathews Estelle Mountain Reserve.
The Lake Mathews area is an important bird resting and feeding site, particularly in the winter months. In addition to a variety of ducks, double-crested cormorants, Western grebe, eared grebe, golden eagles, and bald eagles are present during the winter.
There are no public recreational facilities at Lake Mathews.