Evacuation orders affecting 188,000 people are in place until there is more information on the condition of the US's tallest dam's emergency overflow channel, a sheriff in California said.
There are a lot of unknowns about the situation at the Oroville Dam, about 150 miles north east of San Francisco, Butte County Sheriff Kory Honea said.
A lot of people had to be displaced to ensure public safety and that continues to be his focus, he said.
California fire and rescue chief Kim Zagaris said at least 250 police officers from throughout the state are in the area or en route.
Lake Oroville is about 150 miles north east of San Francisco, and the 770-foot-tall Oroville Dam is the nation's tallest.
Authorities mentioned a plan to plug the hole by using helicopters to drop rocks into the crevasse.
Bill Croyle, the acting director at the department of water resources, said at a news conference that no corrective measures were taken after looking at the flow and available resources.
He said officials will be able to assess the damage to the emergency overflow now that the water is no longer spilling over the top.
He added that the integrity of the dam has not been impacted.