San Andreas and the Linear Lakes of California’s I-280

The 7.44 million people of the San Francisco Bay Area live surrounded by several large faults within the active San Andreas Fault System. Most of us who live here are very aware of this and are even survivors of the 1989 Loma Prieta Earthquake centered near Santa Cruz. We know the next big quake is a huge potential bummer that will one day happen. Some of us prepare with stored food, water and first aid, some don’t — but we all get over the fear and live our lives. The evidence of the faults and the impact of millions of years of plates grinding past each other is pretty easy to find if you go looking. My favorite proof lies along a beautiful stretch of Interstate 280 in San Mateo County.

Linear For A Reason

There is a sign along I-280 that declares it as the World’s Most Beautiful Highway (see it here). While I-280 does not make Fox News’ list of the World’s 10 most beautiful Highways, I agree with the sign. I often commute on this highway and am always amazed at the rolling greenery and sparse development along this freeway that serves so many. One beautiful and very striking feature is a series of long, linear lakes that parallel I-280 for about 7 miles. Their shape is no coincidence. The main trace of the San Andreas Fault runs right under these lakes.

There are two bodies of water that clearly define the main trace of the San Andreas Fault, San Andreas Lake and the Crystal Springs Reservoir. Both were created in the late 1800’s by the Spring Valley Water Company to supply drinking water to San Francisco. The first dam, constructed in 1869, created San Andreas Lake which had been a small natural lake known as a sag pond. Sag ponds form along fault zones because the grinding of the rocks over millennia make the bedrock less permeable, allowing water to pond. However, these reservoirs were built long before anyone knew what a sag pond was or about the San Andreas Fault System and its impacts!

Crystal Springs Reservoir contains two dams. The first one was built in 1877 to create Upper Crystal Springs Reservoir and the second was built in 1888 to create Lower Crystal Springs Reservoir. The second dam, however, effectively impounds water from the watersheds of both lakes so the first dam became a causeway (today’s Highway 92) between the two lakes. In 1924, culverts were installed through this first dam to allow water to flow freely between the upper and lower reservoirs.

Water From the Sierra!

Beyond being clear harbingers of a major fault system these two lakes are also unique because most of the water they store doesn’t come from within their natural watersheds. They are the end of the line for water transported via the 167-mile Hetch Hetchy Aqueduct. Completed in 1934, the aqueduct moves water from the peaks of Yosemite National Park in the high Sierra, across the Central Valley and ends by putting water into Crystal Springs Reservoir at the site of the Pulgas Water Temple (click for a great video and history). The two lakes are connected by tunnels and pumps so water from Hetch Hetchy can be stored in Crystal Springs Reservoir or San Andreas Lake.

Dams Survive Two Big Quakes!

Built in the 19th century with no knowledge of the mighty fault beneath, and on top of some rather fractured bedrock, all three dams in this reservoir system survived the large magnitude 1906 San Francisco and 1989 Loma Prieta Earthquakes. The 1906 quake actively cut through Crystal Springs Reservoir, San Andreas Lake, and the stream valley that connects the two bodies of water. The oldest Upper Crystal Springs Dam (current Highway 92) was actually offset 8 feet (the west side went north) but, amazingly, held. The offset has been removed by repairs over the years. The Lower Crystal Springs Reservoir was undamaged even though the fault rupture was less than 1,000 feet away. Finally, at the San Andreas Dam there was 8 feet of movement immediately to the east of the dam, again with no compromise to the dam! The fact that these three dams survived so close to the 1906 rupture zone and then again survived the 1989 Loma Prieta Quake when they were all over 100 years makes these lakes even more incredible.

San Andrés Forgive Us Our Misspellings

Perhaps you have noted that San Andreas Lake has the same name of the fault I am making so much of? Again this is no coincidence! The San Andreas Fault takes its name from the very same lake. When the Spring Valley Water Company built the San Andreas Dam in 1869, the dam, reservoir and lake were called San Andrés. The name derives from the first Spaniards in Alta California. Gaspar de Portola’s expedition camped above present day San Andreas Lake on November 4th, 1769. Four years later on November 30th, 1773, several from the expedition returned to the site and named the area Cañada de San Andrés after the name of the saint whose feast day it was. The name became established and was seen on late 18th century Spanish maps of the area.

Corruption of the Spanish spelling seems to have started around the 1850s with the change from San Andrés to San Andreas. The accent on the e was intermittently (though often) used through the 1890s and even had occasional use from holdouts through the mid-1960s. The main switch seems to be due to University of California geology professor Andrew Cowper Lawson, who started calling the valley that drained San Mateo Creek the the San Andreas Valley in a published paper in 1893. He was a respected geologist and trained generations of students about what came to be called the San Andreas Rift Valley after the clear offsets from the 1906 quake.

Further Information

If you want to take a hike between San Andreas Lake and Crystal Springs Reservoir and see evidence of movement from the 1906 quake check out the book Geologic Trips: San Francisco and the Bay Area by Ted Konigsmark. He has a guide to where to hike and what to look for. The book, along with other great geologic tours can be found at his website Geologic Trips.