Oroville Dam Earthquake Study Recommended

California officials should conduct an earthquake safety assessment of a Northern California dam that is the linchpin of a major drinking water system in the state, according to a federal inspection obtained by a newspaper.

The Oroville Dam, the tallest in the nation, shows no significant flaws, but inspectors are recommending the assessment because of new information about earthquake hazards in the area, the Sacramento Bee reported on Friday.

The 742-foot-high dam on the Feather River forms the main storage facility for the State Water Project, which supplies drinking water to more than 23 million California residents.

The recommendation was included in a 2010 report by consultants working for the Federal Energy Regulatory Commission, which oversees hydroelectric dams in the United States. The Bee said it reviewed a copy of the report following a Public Records Act request.

But state water officials say they do not plan to conduct the review. The dam is sufficiently strong, and the cost of the review would not be justified, they say.

“Even with today’s understanding of seismicity and ground motions, Oroville still would meet the criteria that would be set today,” said David Panec, chief of dam safety at the state Department of Water Resources’ operations and maintenance division. “The dam is essentially overbuilt.”

The dam – completed in 1968 – has enough water to serve 7 million average households for a year. Its failure would flood numerous cities downstream.

The dam lies just north of a fault line first revealed after a 5.7-magnitude earthquake in 1975. That quake did not damage the dam.

At issue in the inspectors’ report are two other earthquake sources in the area, the Foothill Fault System and the Prairie Creek Fault, according to the Bee.

University of California, Davis, civil engineering professor Ross Boulanger said the computer modeling the inspectors are recommending could cost several hundred thousand dollars.

The Federal Energy Regulatory Commission or California’s Division of Safety of Dams could still require the Department of Water Resources to conduct the study.

Dave Gutierrez, chief of the state’s dam safety division, said he expects his agency will decide in January whether to order further earthquake studies.