Stone Dam, established in 1871, is located in the Pilarcitos Creek Watershed. It was built for the same purpose and about two miles downstream from Pilarcitos Dam. The purpose of both, to collect and send water to a thirsty San Francisco.

The Dams were built by Spring Valley Water Company (known as Spring Valley Water Works at the time). Prior to the SVWC acquisition of the land (a large portion of the watershed, now known as the Upper Pilarcitos Watershed), the land was owned by the enterprising Halsteadt brothers. They owned and ran a grain mill, using the creek to drive their thirty-two foot waterwheel.

Unlike Pilarcitos Dam, which impounds a vast amount of water, Stone Dam is a diversion dam. Diversion dams are typically not used to impound water, but rather to divert all or a portion of a waterway for additional purposing. For instance, the diverted water may be used for irrigation, for the generation of hydro-power, or to be directed into a reservoir. Overflow water from Stone Dam can flow into the Lower Crystal Springs Reservoir via Stone Dam Aqueduct (AKA Stone Dam Tunnels Numbers 1 and 2).

Stone Dam is constructed of rubble masonry, granite blocks quarried below the dam site, and topped with brick, laid herring-bone fashion. It is a thin-arch dam. Its small reservoir has a capacity of five million gallons (15.4 acre feet).

So Pilarcitos Creek is now diverted via Pilarcitos Reservoir and the Stone Dam Aqueduct (Stone Dam Tunnels 1 and 2) into San Mateo Creek and Lower Crystal Springs Reservoir. The Coastside County Water District, which serves Half Moon Bay and other seaside communities, also takes delivery by gravity of up to 2.5 million gallons per day of raw water supplies via this same route from Pilarcitos Reservoir.

So what happens to Pilacitos Creek? “Until recently, no intentional releases were made from Stone Dam to Pilarcitos Creek, and flow in the creek immediately below the dam consisted only of leakage through the spillway boards and seepage through the dam. Currently, experimental releases of a few cubic feet per second (cfs) are being made as part of a study of aquatic resources. In the wet months of wet years, spills over Stone Dam to Pilarcitos Creek are frequent. A tributary adds water to Pilarcitos Creek about one-tenth of a mile below Stone Dam in all but a few months of the driest years.”