Diamond Valley Lake, Artificial reservoir in Hemet, California

Diamond Valley Lake is an artificial reservoir in Riverside County, California, covering more than 1,800 hectares of surface area and reaching depths of up to 79 meters (260 feet). The basin is enclosed by three large earth-fill dams and stores nearly a billion cubic meters of water for emergency use and regional supply.

Planning for the reservoir began in 1987 to meet Southern California's drought, summer-peak and emergency needs. The Metropolitan Water District of Southern California started the $1.9 billion project in 1995; the three earth-fill dams were finished in 1999. Filling via the Colorado River Aqueduct ran from 1999 to 2003.

The lake takes its name from the Diamond and Domenigoni valleys it occupies. Excavation of core material for the dams turned up many paleontological finds, and these are displayed at the Western Science Center at the lake's east end. The reservoir is open to boating, fishing and hiking.

Visitors can hike, cycle and fish around the reservoir, with some shore areas accessible to the public. The easiest way to reach it is by car, as the site sits away from major towns.

The construction site uncovered remains of mammoths and mastodons, now on display at the nearby museum. Boats may only use electric motors to preserve water quality for drinking supply.