Crystal Springs Reservoir is a pair of artificial lakes located in the northern Santa Cruz Mountains of San Mateo County, California situated in the rift valley created by the San Andreas Fault just to the west of the cities of San Mateo and Hillsborough, and I-280. The entire reservoir consists of two different reservoir lakes, split by an earthen dam on which Highway 92 crosses the reservoir. The southern reservoir, Upper Crystal Springs Reservoir, is dammed by the earth dam. It is fed at the south end by a largely submerged tributary, Laguna Creek (or Lake Creek).[1] The northern reservoir, Lower Crystal Springs Reservoir, is fed by San Mateo Creek and San Andreas Creek at its north end. Lower Crystal Springs Reservoir was formed when San Mateo Creek was dammed by the Crystal Springs Dam (San Mateo County, California), which discharges to lower San Mateo Creek. http://en.wikipedia.org/wiki/Crystal_Springs_Reservoir