OROVILLE, Calif., Feb 12 - Residents below the tallest dam in the United States, near Oroville in Northern California, were urgently ordered to evacuate on Sunday after a spillway appeared for a time to be in danger of imminent collapse.

The abrupt evacuation orders came as authorities said that a crumbling auxiliary spillway on the Lake Oroville Dam could give way, unleashing floodwaters onto 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 (0030 GMT Monday) that the spillway next to the dam was "predicted to fail within the next hour."

Several hours later the situation appeared less dire as the spillway remained standing some five hours later and the Water Resources department said crews using helicopters would drop rocks to fill a 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.

Butte County Sheriff Korey Honea said at an evening press conference 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.

But he said he was told later that the erosion was not progressing as rapidly as earlier feared and that the amount of water flowing over the spillway had dropped quickly.

Still, evacuation orders remained in place. 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.

Oroville itself was largely deserted after nightfall.

It is not clear how many people were affected by the evacuation order. More than 160,000 people live in the evacuation area comprising three counties, according to U.S. Census data.

The Oroville dam is nearly full after a wave of winter storms 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.

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

The earth fill 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).