OROVILLE (California) • Emergency crews yesterday prepared loads of rock to be dropped by helicopters to seal a crumbling spillway, which threatens to inundate communities along the Feather River in northern California, local media reported.

A day earlier, almost 200,000 people were ordered to evacuate from the area below the Lake Oroville Dam, the tallest dam in the US, after the 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 state Emergency Services Office and Department of Water Resources did not immediately respond to requests for comment.

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

The department said the authorities were releasing water to lower the lake's level after weeks of heavy rain 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 that "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. At 230m high, the structure, built between 1962 and 1968, is the tallest US dam, exceeding the Hoover Dam by more than 12m.

Officials said they feared the damaged spillway could unleash a 9m-high wall of water on Oroville.

REUTERS