Crystal Springs Dam

Crystal Springs Dam is a concrete gravity dam constructed across the San Mateo Creek in San Mateo County, California, United States. It impounds water in a rift valley created by the San Andreas Fault to form the Crystal Springs Reservoir. The dam is located about 1,100 feet (340 m) east of the fault. It 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 145 feet (44 m) high with a crest length of 600 feet (180 m). It was the first mass concrete gravity dam built in the United States. Upon its completion in 1889, it became the largest concrete structure in the world and the tallest dam in the United States. A 2024 review by American Society of Civil Engineers (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 ASCE. The dam has undergone multiple repairs and improvements throughout its lifetime, and survived both the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake with minimal damage.

Spring Valley Water Works, the private utilities corporation that eventually developed the Crystal Springs Dam, was formed in 1858 to provide the rapidly expanding port city of San Francisco with a reliable water source. Under the leadership of George Ensign, a prominent land baron, it quickly established a monopoly on the San Francisco water market. Understanding that any potential water source would have to come from outside of the naturally barren city limits, Ensign turned towards adjacent San Mateo County, along with its largely undeveloped mountains and streams. Over the next three decades, Spring Valley Water Works constructed a variety of conduits, dams, and tunnels to connect the San Andreas Valley watershed to San Francisco. Despite this, there was still a desire for a larger reservoir to ensure that San Francisco had a more reliable long-term water source. Hermann Schussler, a German immigrant who moved to California in 1864 as a civil engineer, quickly rose through the ranks to become Chief Engineer of the Spring Valley Water Company. Cognizant of the growing desire for a reservoir, Schussler encouraged the company to acquire as much watershed property as possible. By the time Schussler began considering the development of a dam to impound the San Mateo Creek, the company had already acquired over 100,000 acres (40,000 ha) of prime property across the San Francisco Peninsula. The location of the proposed dam site had its benefits and disadvantages: located in a 700 feet (210 m) valley formed by San Mateo Creek, sandstone cliff walls rose over 200 feet (61 m) from the creekbed. The dam encloses a larger valley that spreads outwards both east and west of the dam itself. Schussler's initial design created a lake approximately 9 miles (14 km) long. Geological testing was conducted using boring techniques, which showed that the valley was favorable for the dam's construction: the foundation was hard sandstone that contained no cracks or fissures.

Dam data from Wikidata (CC0) plus curated additions. Summary from Wikipedia (CC BY-SA).