By Dan Whitcomb

(Reuters) – Residents below the tallest dam in the United States, near Oroville in Northern California, were urgently ordered to evacuate on Sunday as a spillway appeared in danger of imminent collapse.

The abrupt evacuation orders came as authorities determined that the auxiliary spillway on the Lake Oroville Dam could give way at any time, 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 Oroville dam is nearly full after weeks of heavy rains and snow brought relief to the state after some four years of devastating drought.

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.”

However it was still standing nearly three hours later as the Water Resources department said crews would use helicopters to drop rocks to fill a gouge in the spillway. Authorities were also releasing water to lower the lake’s level.

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 roads leading out of the area were jammed as residents sought to drive out of the flood zone.

It was not clear how many residents were affected by the evacuation order.

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.

Water levels were less than 7 feet (2 meters) from the top of the dam on Friday.

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 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).

(Reporting by Dan Whitcomb in Los Angeles; Editing by Peter Cooney and Mary Milliken)