Nearly 200,000 people living below the tallest dam in the United States, near Oroville in Northern California, were urgently ordered to flee their homes on Sunday after a spillway appeared for a time to be in danger of imminent collapse.

Authorities issued the abrupt evacuation orders in the mid-afternoon, saying that a crumbling emergency spillway on the Lake Oroville Dam could give way and unleash raging floodwaters onto a string of rural communities along the Feather River. "Immediate evacuation from the low levels of Oroville and areas downstream is ordered," the Butte County sheriff said in a statement posted on social media. "This is NOT A Drill."

The California Department of Water Resources said on Twitter at about 4:30 p.m. PST that the spillway next to the dam was "predicted to fail within the next hour."

But several hours later the situation appeared less dire as the spillway remained standing.

The water resources department said crews using helicopters would drop rocks to fill a huge gouge in the spillway. Authorities were also releasing water to lower the lake's level after weeks of heavy rains in the drought-plagued state.

By 10 p.m., state and local officials said those efforts had paid off and, with water no longer flowing over the eroded spillway, the immediate danger had passed. But they cautioned that the situation remained unpredictable.

"Once you have damage to a structure like that it's catastrophic," Bill Croyle, acting director of the Water Resources, told the press conference.

'DO NOT TRAVEL NORTH'

Butte County Sheriff Korey Honea said at a separate news briefing that he was told by experts earlier on Sunday that the hole that was being created in the spillway could compromise the structure. Rather than risk thousands of lives, the sheriff said, a decision was made to order the evacuations.

Still, evacuation orders remained in place for 188,000 people in Oroville, Yuba County, Butte County, Marysville and nearby communities.

The Yuba County Office of Emergency Services urged evacuees to travel only to the east, south or west. "DO NOT TRAVEL NORTH TOWARD OROVILLE," the department said on Twitter.

Evacuation centers were set up at a fairgrounds in Chico, California, about 20 miles northwest of Oroville, but major highways leading south out of the area were jammed as residents fled the flood zone.

California Governor Jerry Brown issued an emergency order that he said would bolster the state's response.

"I've been in close contact with emergency personnel managing the situation in Oroville throughout the weekend and it's clear the circumstances are complex and rapidly changing," Brown said.

The Oroville dam, whose structure remains sound, is nearly full following a wave of winter storms that brought relief to the state after some four years of devastating drought. Water levels were less than 7 feet (2 meters) from the top of the dam on Friday.

State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam some 65 miles (105 km) north of Sacramento after noticing that large chunks of concrete were missing from a spillway.

Governor Brown asked the Federal Emergency Management Agency on Friday to declare the area a major disaster due to flooding and mudslides brought on by the storms.

The earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people.

At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest dam in the United States, besting the famed Hoover Dam by more than 40 feet (12 meters).