The dam is the tallest in the United States, exceeding even Hoover Dam. It stands 770 feet, 44 feet taller than Hoover. Lake Oroville is the second-largest reservoir in the state, behind Shasta Lake, which is the center of the federal water project that serves California.

Flow from heavy rains has damaged the reservoir’s spillway, creating dramatic scenes of muddy torrents gushing over a concrete chute marred by a massive sinkhole.

Officials have stressed that the dam itself suffered no damage and that the spillway problems don’t pose a imminent threat to the public. Still, they have been frantically working to reduce the amount of water in the Lake Oroville reservoir, which is near capacity. It’s now at about 96% of capacity, and more water has been flowing in than is draining out.

If the reservoir exceeds capacity, it could damage the dam. Officials plan to avoid that, if necessary, by using an emergency spillway to release water. It’s unclear whether they will have to do that.