Crystal Springs Reservoir is an artificial lake 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 SR 92 crosses the reservoir. The southern reservoir, Upper Crystal Springs Reservoir, is dammed by the earth dam. The northern reservoir, Lower Crystal Springs Reservoir, is dammed by the Crystal Springs Dam, which discharges to San Mateo Creek. Part of the water in the reservoir comes from local precipitation and the rest is piped in from the Hetch Hetchy Reservoir in Yosemite National Park. The entire reservoir was built and owned by a private company, in the form of the Spring Valley Water Company, and eventually was deeded under the ownership and protection of the city of San Francisco. This local protection has ensured the survival of important species in the area, and a set of trails in Crystal Springs Park allows many to enjoy the scenic beauty of the reservoir and the local wildlife.