Almost 200,000 people were ordered on Sunday 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 with backhoes. 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 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 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. At 770 feet (230 meters) high, the structure, built between 1962 and 1968, is the tallest US dam, exceeding the Hoover Dam by more than 40 feet (12 meters).
'Do not travel north'
Officials said they feared the damaged spillway could unleash a 30-foot (9-meter) wall of water on Oroville. They said evacuation orders remained in place for some 188,000 people in the area and would be re-evaluated at dawn.
The Yuba County Office of Emergency Services urged evacuees to travel east, south or west. "DO NOT TRAVEL NORTH TOWARD OROVILLE," the department warned on Twitter.
Evacuation centers were set up at a fairgrounds in Chico, California, about 20 miles (32 km) northwest of Oroville, but major highways leading south out of the area were jammed as residents fled the flood zone.
Javier Santiago, 42, fled with his wife, two children and several friends to the Oroville Dam Visitors Center in a public park above the dam and the danger zone.
With blankets, pillows and a little food, Santiago said: "We're going to sleep in the car."
The Oroville dam is nearly full following winter storms that brought relief to the state after four years of drought.
State authorities and engineers on Thursday began releasing water from the dam after noticing that large chunks of concrete were missing from the spillway.