High Quality Content by WIKIPEDIA articles! Lake Mission Viejo is a reservoir entirely within Mission Viejo, in Orange County in the U.S. state of California. About 5 miles (8.0 km) south of Santiago Peak, a peak in the Santa Ana Mountains, the reservoir is formed by an earthfill dam across the canyon of Oso Creek, which is part of the Trabuco Creek and San Juan Creek drainage basin. The lake is not fed by urban runoff, it is maintained so as to be safe for contact. Two man-made beaches are located on the north and east ends of the lake. The lake is annually stocked with fish.