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What is most surprising about the near-collapse of a spillway at Oroville Dam is that events like that have not happened more often. Located 75 miles north of Sacramento, Oroville is the nation’s tallest dam and holds back the state’s second-largest reservoir. Both its main spillway and an auxiliary one have experienced major erosion because of massive emergency releases of reservoir water during this winter’s heavy storms. On Sunday, engineers worried that the top 20 or 30 feet of the emergency spillway could give way, causing devastating flooding on the Feather River.

A powerful new storm is expected to arrive in Southern California on Friday, and it could provide a walloping, with possible flash flooding, mudslides and rock slides. “The Friday storm in particular could in fact become the strongest of the season in the Los Angeles region,” said UCLA climate scientist Daniel Swain.

A comprehensive condition assessment of nine dams owned by the city of San Diego has been underway for the past year, according to the Public Utilities Department said. Asked about the condition of San Diego dams after Sunday’s mass evacuation in Oroville in Northern California, department officials told City News Service that they hired independent experts in dam design, construction and safety to perform detailed inspections of the dams in February of last year.

A year ago, politicians and experts were predicting a near-permanent statewide drought, a “new normal” desert climate. The most vivid example of how wrong they were is that California’s majestic Oroville Dam is currently in danger of spillway failure in a season of record snow and rainfall. That could spell catastrophe for thousands who live below it and for the state of California at large that depends on its stored water.

The game plan is to get water behind the Oroville Dam below what its engineering designs call “flood control storage,” and keep it there. At that depth, the dam would have a buffer capacity of half a million acre-feet of water. At the current release rate, a pounding 100,000 cubic feet per second, the dam will reach that point by late Saturday or early Sunday, even with another rain system arriving Wednesday, said Bill Croyle, acting director of the state Department of Water Resources.