The San Diego River is a river in San Diego County, California. It originates in the Cuyamaca Mountains northwest of the town of Julian, then flows to the southwest until it reaches the El Capitan Reservoir, the second-largest reservoir in the river's watershed at 112,800 acre-feet (139,100,000 m3). Below El Capitan Dam, the river runs west through Santee and San Diego. While passing through Tierrasanta it goes through Mission Trails Regional Park, one of the largest urban parks in America.