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. … In the six years since Oroville, dam operators across the country have begun to reassess 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 it may not be enough to prevent the next disaster. Bigger storms are on the way.

Related article: 

• Newsweek: Before and after photos show Lake Oroville go from empty to full capacity