Features & Columns

SHAKY GROUND: A neighborhood in the shadow of Anderson Dam, which could fail in the event of a major earthquake.

In February 2017, the Anderson Reservoir spilled over. That month, torrential storms sent a deluge of water into many low-lying neighborhoods near Coyote Creek. The resulting floods forced 14,000 people from their homes and did $100 million in damage.

If Anderson Dam were to fail suddenly, the fallout would be catastrophic. A breach of Anderson Dam at full capacity could inundate surrounding land more than 30 miles northwest to San Francisco Bay, and more than 40 miles southeast to Monterey Bay.

On the 30th anniversary of the Loma Prieta earthquake, and with another typical winter rainy season—the third in a row—approaching, public water experts are accelerating their efforts to improve the capacity and stability of Anderson Dam and the Anderson Reservoir, Santa Clara County's largest body of water.

The 235-foot-high earthen dam measures 1,430 feet long by 900 feet wide and sits along the Coyote Creek Fault on Coyote Road, east of Morgan Hill. The reservoir itself is situated parallel to the Calaveras Fault, which runs from Hollister to Milpitas. It holds over 90,000 acre feet of water when full, more than the other nine reservoirs in the county combined.

The Santa Clara Valley Water District, which now calls itself Valley Water, says it hopes to break ground on a five-year, $550 million project to upgrade the earthquake safety of the Anderson Dam in 2021. The popular recreation lake would be drained for at least five years during the project.

New seismic data in 2018 prompted the district, which owns the reservoir, to revise and expand its plans for the Anderson Seismic Retrofit, boosting the cost and timetable for the project. According to the seismic study, the nearly 70-year-old earthen dam needs to be rebuilt.

Recent geologic investigations of the dam have uncovered "previously unidentified seismic deficiencies." The most alarming of the investigation's conclusions might be that the upstream embankment is "susceptible to liquefaction" during a "maximum considered earthquake."