Residents living near the Oroville Dam in northern California have been evacuated as the facility nears collapse after heavy rainfall.
More than 180,000 people have been asked to leave as officials say the emergency spillway of the 770ft dam is close to collapse.
After years of severe drought, water levels in the reservoir rose after heavy rain and snow, the BBC said.
The California Department of Water Resources said it was releasing as much as 100,000 cubic feet of water per second from the main spillway, in an attempt to lower the level of the lake and relieve pressure.
Helicopters were also dispatched and dropped boulders to try to block the eroded area next to the spillway.
Although excess water has now stopped flowing, the evacuation order is still in place.
This is the first time that there has been such an emergency at the dam in its 50-year history.
Residents were told to head north, causing gridlock on roads, with some complaining warnings should have been given earlier.
Emergency shelters have been provided as many hotels are fully booked in the area.