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

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.