Lake Mathews

Lake Mathews
A view of Lake Mathews from a distance.
Location	Riverside County, California
Coordinates	33°50′25″N 117°26′07″W / 33.8403°N 117.4352°W / 33.8403; -117.4352Coordinates: 33°50′25″N 117°26′07″W / 33.8403°N 117.4352°W / 33.8403; -117.4352
Type	reservoir
Primary inflows	Colorado River Aqueduct
Primary outflows	Pipelines to member water agencies in the greater Los Angeles area
Basin countries	United States
Max. length	4 mi (6.4 km)
Max. width	2 mi (3.2 km)
Average depth	Approx. 150 ft (46 m)
Water volume	182,000 acre feet (224 hm3)
Shore length1	Approx. 14 mi (23 km)
Surface elevation	1,391 ft (424 m)
Islands	several small islands; number present depends on reservoir level
1 Shore length is not a well-defined measure.

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. These roads are often used by commuters as a shortcut between Interstate 15 and Interstate 215 or California 91.

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.

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, at the same time 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 to construct a new tower as an alternate. 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 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 (Dipodomys stephensi) in the area. The area is now called the Lake Mathews Estelle Mountain Reserve.