Heavy rains in the drought-plagued state have caused such danger

About 200,000 people living below the tallest dam in the United States, near Oroville in 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 midafternoon, 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 pm 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 pm, 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 news conference.

'Do not travel north'

Butte County Sheriff Korey Honea said 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.

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 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 100 km north of Sacramento after noticing that large chunks of concrete were missing from a spillway.

At 230 meters high, the structure, built in 1962-68, is the tallest dam in the US, besting the famed Hoover Dam by 12 meters.