On the morning of February 7, 2017, two electricians were working on a warning siren near the Oroville Dam spillway, 60 miles north of Sacramento, when they heard an explosion. As they watched, a giant plume of water rose above their heads, and chunks of concrete began to fly down the mountainside toward the Plume River. The dam’s spillway, a concrete channel capable of moving millions of gallons of water from the reservoir in seconds, crumbled before them. If it were to be removed, a serious downpour, like the one that has been falling on Northern California for days, could cause the dam — the tallest in the United States — to fail.

Kory Honea, the sheriff of Butte County, which includes the dam and the town it’s named after, first heard something was wrong from Dino Corbin, a local radio personality, who called him at his office: “Are you aware there’s a hole in the spillway?” Around the same time, one of the sheriff’s dispatchers received a confusing message from the California Department of Water Resources, which owns the dam, saying it was conducting a “routine inspection” after reports of an incident.

At the dam, department officials closed the gates at the top of the spillway to prevent more of its concrete slabs from being lost in what an independent forensic report prepared after the incident described as a “sudden, explosive failure.” The flow of water stopped. The rain, however, did not.

In the six years since the near-failure of the Oroville Dam, dam operators across the country have begun reassessing the structures under their control, looking for hidden weaknesses: the cracks in the spillway, the hillside that crumbles at the first sign of water. That work is necessary, but may not be enough to prevent the next disaster. Bigger storms are on the way.

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Additional production for The Sunday Read was contributed by Isabella Anderson, Anna Diamond, Sarah Diamond, Elena Hecht, Emma Kehlbeck, Tanya Pérez, and Krish Seenivasan.

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