Construction of Crystal Springs Dam

As they traveled south, they realized the valley sloped slightly and that other water sources drained into the little stream from the west. They noticed a stream entering the small waterway from another valley which they would have to explore later. Yet another stream was observed flowing from the south to the lower part of the valley, and the combined waters flowed to the east through a gap in the hills.

The geology of the Coast Range of California is complicated and difficult to comprehend until certain factors are understood. It had been discovered that the earth slowly moves on huge crustal plates. When the Pacific Plate (moving eastward) encountered the North American Plate (moving westward) millions of years ago, some of the crust of the ocean plate (ocean bottom) scraped off and formed the Coastal Range. It is mainly unconsolidated sandstone and shale called graywacke and is quite different from the sandstone and shale found on the Colorado Plateau in Arizona. It eroded when rains fell on it, forming the rounded hills you see now. Also, another factor has to be included in this geology picture: The San Andreas Fault. This is an area where the two large plates meet and scrape against each other — slowly. Over a period of time, they formed the San Andreas Valley, but Gaspar de Portola did not know this when he traveled through it in 1769. It was just a nice little valley to him.

In time it was realized that the valley would make a great place to store water. It was enclosed by the hills, and it had a good supply of water, as was immediately observed by the early Spanish explorers.

San Mateo Creek was untamed in the 1700s, and this was OK with the Spanish as all they wanted was to grow crops on the open flat areas around it. Very few people other than the Native Americans lived in the area. The marshes that formed as the creek flowed toward the Bay didn’t concern them as there was plenty of other land to use. Ducks and other birds by the millions used the edge of the Bay for feeding and breeding. When the Americans came to settle the Peninsula in the 1850s, however, the wild waters of the creek were not welcome.

The construction of a dam in the San Andreas Valley in 1868 was the beginning of the end for the creek’s unruly behavior. This dam controlled the water runoff in the northern section of the San Andreas Valley.

By 1870, Alvinza Hayward had attempted to supply water for the city by building a dam on the San Mateo Creek. He constructed the first municipal reservoir. But the Spring Valley Water Company had different plans for the water being supplied by the creek.

In 1875, another dam was begun due west of Belmont on the Laguna Creek. This earthen dam controlled the water flow from the southern area. This dam is now used as part of State Route 92.

The final step in controlling the flow of water that drained the valley formed by the San Andreas Fault came in 1887 when the Spring Valley Water Company of San Francisco began building a dam at the gap in the hills where the San Mateo Creek flowed toward the city of San Mateo. In 1889, the projected height of the dam reached 120 feet and although it was thought it would take a decade to fill a new lake, it took less than a year. An especially wet year filled it up fast, and the water flowed over the top. Another 25 feet of cement was added to the dam to control this in the future.

This engineering marvel, designed by German immigrant Hermann Schussler, the Crystal Springs Dam, has withstood the ravages of time as well as many earthquakes that destroyed other man-made structures. Although the object of the dam was to supply water to San Francisco and the Peninsula, it also tamed one of the big threats of Mother Nature to peaceful development of the cities to the east of its canyon. No longer would its uncontrolled waters gouge out the canyon walls or flood onto the plain below on its way to the Bay.