Crystal Springs Dam, Gravity dam in San Mateo County, US.

Crystal Springs Dam is a concrete gravity dam 145 feet (44 m) high with a crest length of 600 feet (180 m), built across San Mateo Creek. It impounds the Crystal Springs Reservoir in a rift valley of the San Andreas Fault. The dam consists of interlocking concrete blocks with staggered joints, and Skyline Boulevard runs over its crest.

Crystal Springs Dam was completed in 1889 at a height of 120 feet, making it the tallest dam in the United States and the largest concrete structure in the world at the time. It was built by the Spring Valley Water Works under chief engineer Hermann Schussler to supply San Francisco with drinking water. A wet winter in 1889–1890 filled the reservoir in one year, prompting Schussler to plan raising the dam to 145 feet.

The dam's name comes from the Crystal Springs estate of the Spring Valley Water Works, which bought watershed land across the San Francisco Peninsula from 1858 onward. Its engineer, Hermann Schussler, a German immigrant who arrived in California in 1864, had the concrete blocks poured to a fixed water-to-cement ratio of 0.45. Visitors cross the crest on Skyline Boulevard, where the Sawyer Camp Trail begins.

The location is easily reached via Skyline Boulevard, which passes directly over the structure and allows vehicle access to the area. The site also serves as a trailhead for hiking and biking activities along the scenic Sawyer Camp Trail.

The dam's blocks were poured in interlocking 7–9 foot sections with staggered joints, so no horizontal or vertical joint lines up. This lets thousands of separate blocks behave as one mass. Schussler specified a water-to-cement ratio of 0.45 for every block, and the concrete was mixed with Portland cement shipped from Swanscombe in England and sand carried by boat from San Francisco's North Beach.