Don Castro Regional Park is a 101-acre urban oasis on the boundary between Hayward and Castro Valley. The clear blue waters of the swim lagoon are a favorite with central Alameda County residents. The lagoon and adjacent lake were created in 1964 when the Alameda Flood Control and Water Conservation District dammed San Lorenzo Creek.

Swimming in the lagoon is the park's main attraction. There is a beach access fee. Swim hours are from 11 a.m. to 6 p.m., and only when lifeguards are on duty.