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Crystal Springs Dam is a gravity dam constructed across the San Mateo Creek in San Mateo County, California. It impounds water to form the Lower Crystal Springs Reservoir which sits atop the San Andreas Fault in a rift valley created by the fault. The dam itself is located about 300 yards (273 meters) east of the fault.[1][2] It was among the first concrete gravity dams built in the western United States.[3] Skyline Boulevard runs over the dam, which also forms the trailhead of Sawyer Camp Trail.
| Crystal Springs Dam | |
|---|---|
 | |
| Official name | Lower Crystal Springs Dam |
| Location | San Mateo County, California |
| Coordinates | 37.5285°N 122.3622°W |
| Opening date | 1888 |
| Operator(s) | San Francisco Public Utilities Commission |
| Dam and spillways | |
| Impounds | San Mateo Creek |
| Height | 140 ft (43 m) |
| Length | 600 ft (180 m) |
| Width (base) | 40 ft (12 m) |
| Reservoir | |
| Creates | Lower Crystal Springs Reservoir |
| Total capacity | 57,910 acre⋅ft (71,430,000 m3) |
| Catchment area | 29.4 sq mi (76 km2) |
| Surface area | 1,323 acres (5.35 km2) |
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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.[4] Quickly understanding that any potential water source would have to come 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.[5]
Hermann Schussler, a German immigrant who moved to California in 1864 as a prospective civil engineer, quickly rose 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 (400 km2) of prime property across the San Francisco Peninsula. [5]
The location of the proposed dam site had its benefits and downfalls: 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.[6] Geological testing was conducted using boring techniques, which showed that the valley was favorable towards the dam's construction: the foundation was hard blue sandstone that contained no cracks nor fissures.
Schussler's designs called for a projected height of 140 feet (43 m). At the time there were only 17 constructed dams in the world taller than 95 feet (29 m), and all were located outside the United States. A majority of these dams were either rock-filled or earth-filled dams. This approach, however, could not work for Schussler's design, as there were no suitable quarries to procure materials from.[5] However, during the search for quarry sites, a large outcrop of indurated sandstone was located, especially conductive for concrete. As such, Schussler switched to a design using mass concrete blocks.[7]
At the time, the United States had no large-scale manufacturer of cement, leading to Portland cement being transported via ship from Swanscombe,England,[5] which at the time was the biggest Portland cement manufacturing plant in the United Kingdom.[8] The dam was constructed by separately pouring large blocks of the structure in place, and allowing them to set before pouring the adjoining blocks. This enabled the bricks to set and harden individually, while also ensuring that the concrete would not crack during the curing process. An important design feature is that neither the horizontal nor the vertical joints line up. This helps the structure act as a single monolithic construction despite being made of thousands of individual blocks, while also enhancing its structural stability.[6]
Schussler ensured that each block of the dam was poured with concrete made to the exact proportions of his mix specifications.
The structure was completed in 1888 at a height of 120 feet (37 m).[5] However, a wet winter in 1889-1890 caused the dam to overflow, leading to an addition to bring the total height of the dam to 145 feet (44 m), making it the tallest dam in the United States at the time.[6] At the time of its completion, with 4,239,000 cubic feet (120,000 m3) of concrete,[5] it was the largest concrete structure in the world.[9]
In 1976, the dam was designated as a California Historic Civil Engineering Landmark by the American Society of Civil Engineers (ASCE).[10][11] The dam was later designated as a National Historic Civil Engineering Landmark by ASCE in 2023.[12]
The dam survived both the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake with only minor damage – despite its location about 300 yards (270 m) east of the San Andreas Fault. It was subject to severe shaking in both earthquakes.[3]
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The San Francisco Public Utilities Commission began the process of renovating the dam in 2003. A major step in the renovation process, doubling the width of the main spillway and raising the dam to increase the water storage capacity, was completed in 2012. The renovations, which are intended to improve the reliability of the system in the event of an earthquake, were completed in 2016. Skyline Boulevard, the road on top of the dam, was closed for the construction of a new replacement bridge in October 2010. Along with the replacement bridge, the parapet wall was raised by nine feet, the dam's spillway was widened, and power lines were relocated to the underside of the bridge. Construction completed in late 2018 and the roadway was reopened in January 2019.[13][14]
Crystal Springs Dam from downstream
Lower Crystal Springs Reservoir
Aerial view of the Crystal Springs Reservoir from the southeast
View from under the California State Route 35 roadway that was added atop the dam
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