On the morning of Feb. 7, 2017, two electricians were working on a warning siren near the spillway of Oroville Dam, 60 miles north of Sacramento, when they heard an explosion. As they watched, a giant plume of water rose over their heads, and chunks of concrete began flying down the hillside toward the Feather River. The dam’s spillway, a concrete channel capable of moving millions of gallons of water out of the reservoir in seconds, was disintegrating in front of them. If it had to be taken out of service, a serious rainstorm, like the one that had 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 is named for, first heard that 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 confusi...