On our way to Harford Springs Reserve in the Gavilan Hills area off Cajalco Road in Riverside County, we passed by Lake Mathews, which looked beautifully calm and serene in the distance. This reservoir, fed by inflows from the Colorado River Aqueduct and surrounded by several thousand acres of protected land, has been closed to public access for decades.
From Wiki: https://en.wikipedia.org/wiki/Lake_Mathews
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.