Emergency crews early prepared loads of rock to be dropped by helicopters to seal a crumbling spillway that threatens to inundate communities along the Feather River in Northern California.

Almost 200,000 people were ordered to evacuate from the area below the Lake Oroville Dam, the tallest dam in the United States, after authorities said its emergency spillway could give way.

Officials said the situation seemed less dire overnight but Sacramento television station KCRA reported that helicopters from around the state were sent to drop chest-high bags of rocks to close the hole in the spillway.

The NBC affiliate showed dump trucks dropping off piles of rock, which were then loaded into the bags.

The operation to close the gap would begin as soon as it was feasible, the station said.

The state Emergency Services Office and Department of Water Resources did not immediately respond to requests for comment.

The water department said on Twitter yesterday that the spillway next to the dam was "predicted to fail within the next hour" but it remained standing.

The department said authorities were releasing water to lower the lake's level after weeks of heavy rains in drought-plagued California.

Overnight, state and local officials said the immediate danger had passed with water no longer flowing over the eroded spillway but they cautioned that the situation remained unpredictable.

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

But he stressed "the integrity of the dam is not impacted" by the damaged spillway.

The earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people north of the state capital of Sacramento.

Asked about the evacuation order, Mr Croyle said "It was a tough call to make."

He added: "It was the right call to make."

Evacuation centres have been set up in Chico, California, about 30km northwest of Oroville, but roads leading out of the area were jammed as residents sought to drive out of the flood zone.

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 US 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 2m from the top of the dam on Friday.

State authorities and engineers on Thursday began carefully releasing water from the Lake Oroville Dam 105km 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 earthfill dam is just upstream and east of Oroville, a city of more than 16,000 people.

At 230m high, the structure, built between 1962 and 1968, is the tallest dam in the United States, higher than the famed Hoover Dam by more than 12m.