Crystal Springs Dam

Crystal Springs Dam is a concrete gravity dam constructed across the San Mateo Creek in San Mateo County, California. It impounds water in a rift valley created by the San Andreas Fault to form the Crystal Springs Reservoir. The dam itself is located about 1,100 feet (340 m) east of the fault. The dam is owned and operated by the San Francisco Public Utilities Commission, and stores drinking water for the City of San Francisco. The current dam is 140 feet high with a crest length of 600 feet.

It was the first mass concrete gravity dam built in the United States. Upon its completion, it became the largest concrete structure in the world and the tallest dam in the United States. A 2024 review by ASCE and the Institution of Civil Engineers in Great Britain indicated that it is likely the oldest mass concrete dam in the world. Construction techniques used at the dam, including washing aggregate and staggered joints, influenced the development of future gravity dams, such as Hoover Dam and Grand Coulee Dam. In December 2023, it was recognized as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers (ASCE).

The dam has undergone multiple repairs and improvements throughout its lifetime, and has survived both the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake with minimal damage, despite being located approximately 1,100 feet (340 m) from the San Andreas Fault.