The State Water Project facility Lake Perris, in northwestern Riverside County, stores water for Inland Empire cities such as San Bernardino and Riverside. [See also Santa Ana River.]

Built between 1970 and 1974, Lake Perris is the SWP’s southernmost reservoir and is one of the most popular recreational lakes in the system. 

Lake Perris also plays a role in the Monterey Agreement. The Agreement between the Department of Water Resources and water contractors provided greater flexibility in water storage in case of droughts. This changed how the lake, along with Castaic Lake, is operated by allowing it to store excess water flows during wet years to help contractors prepare for dry years.