 Below Crystal Springs Dam, lower San Mateo Creek receives limited flows from Lower Crystal Springs Reservoir and descends to the Bay.In 1924, culverts were built through Upper Crystal Springs Dam to hydraulically link Upper and Lower Crystal Springs Reservoirs.Part of the water in the reservoirs comes from local precipitation and the rest is piped in from the Hetch Hetchy Reservoir in Yosemite National Park, as well as the Pilarcitos Creek watershed and Alameda Creek watershed.