Lake Mathews is a large reservoir in Riverside County, California, located in the Cajalco Canyon in the foothills of the Temescal Mountains. It is the western terminus for the Colorado River Aqueduct that provides much of the water used by the cities and water districts of the Metropolitan Water District of Southern California. The reservoir is fenced and closed to all public access. The lake is near the Riverside community of Lake Hills, and is skirted by Cajalco Road on the south, El Sobrante Road on the north and east, and La Sierra Avenue on the west. La Sierra Avenue traverses the top of the west dike. The reservoir was constructed by building a large earthfill dam across the northern side of the basin. Two smaller dams, called dikes, were built during the 1961 expansion to increase the lake's capacity. History Originally named the Cajalco Reservoir, the reservoir is now named after W.B. Mathews, an attorney who was a key architect of the MWD and its business relationships with member water agencies. Construction on the reservoir began in 1933, while the aqueduct was being built across the desert. The reservoir site had previously been occupied chiefly by carob orchards and tin mines. Lawrence Holmes, Sr., who owned 1,100 acres in the future reservoir basin, lost his property to eminent domain in a lengthy court battle. The dam across Cajalco Canyon and its 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. In the mid-2000s, a large project was undertaken to rehabilitate the old outlet tower and construct a new tower as an alternative. The MWD was concerned about the tower's susceptibility to earthquakes, and age had rendered many of the tower's massive valves unusable. To allow for construction without impacting the water supply, a cofferdam was constructed. Massive concrete tunnels were built to connect the new tower to the existing waterworks. Lake Mathews Ecological Reserve 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 Stephen's kangaroo rat (Dipodomys stephensi) in the area. The area is now called the Lake Mathews