Water levels have dropped at California's Lake Oroville, stopping water from spilling over a massive dam's potentially hazardous emergency spillway after authorities ordered the evacuation of nearly 200,000 people from towns lying below the lake.
California department of water resources officials are waiting for the light of dawn to inspect an erosion scar on the spillway at the Oroville Dam, the US's largest.
The evacuations for people living below the lake were ordered on Sunday after authorities warned that failure of the emergency spillway could send a 30ft wall of water into the communities.
The lake that also serves as a reservoir has swelled significantly in recent weeks because California has been hit by a series of storms that have dumped rain and snow across the state, particularly in northern California, where the lake lies about 150 miles north east of San Francisco.
The threat appeared to ease somewhat on Monday, with officials saying water flows into the lake stood at about 45,000 cubic feet per second with outflows at 100,000 cubic feet per second.
Lake Oroville is one of California's largest man-made lakes and had water levels so high on Saturday that its emergency spillway was used for the first time since it was built nearly 50 years ago.
Sunday afternoon's evacuation order came after engineers spotted a hole on the concrete lip of the secondary spillway for the 770ft-tall Oroville Dam and told authorities that it could fail within the hour.
The sudden evacuation panicked residents, who scrambled to get their belongings into cars and then grew angry as they sat in bumper-to-bumper traffic hours after the evacuation order was given.
Kaysi and Greg Levias packed everything they could into their car from their Yuba City apartment and piled everything they were leaving behind as high as possible before leaving the city they moved to just three weeks ago.
"We've never been through this before," said Kaysi Levias. "We have two boys and our dog. All the stuff we could fit in the trunk - clothes and blankets."
Raj Gill, managing a Shell station where anxious motorists got fuel and snacks, said his boss told him to close the station and flee himself.
But he stayed open to feed a steady line of customers.
"You can't even move," he said.
"I'm trying to get out of here too. I'm worried about the flooding. I've seen the pictures - that's a lot of water."
A Red Cross spokeswoman said more than 500 people showed up at an evacuation centre in Chico, California.
The shelter had run out of blankets and cots, and a tractor trailer with 1,000 more cots was stuck in the gridlock of traffic fleeing the potential flooding, said Red Cross shelter manager Pam Deditch.
A California highway patrol spokesman said two planes would fly on Monday to help with traffic control and possible search and rescue missions.
At least 250 California law enforcement officers were posted near the dam and along evacuation routes to manage the exodus of residents and ensure evacuated towns do not face looting or other criminal activity.
"There is still a lot of unknowns," Butte County sheriff Kory Honea said.
"We need to continue to lower the lake levels and we need to give the department of water resources time to fully evaluate the situation so we can make the decision to whether or not it is safe to repopulate the area."
About 188,000 residents of Yuba, Sutter and Butte counties were ordered to evacuate.
Bill Croyle, acting director of the department of water resources, said officials will be able to assess the damage to the emergency spillway now that the lake levels have come down.
The erosion at the head of the emergency spillway threatens to undermine the concrete weir and allow large, uncontrolled releases of water from Lake Oroville.
Those potential flows could overwhelm the Feather River and other downstream waterways, channels and levees and flood towns in three counties.
Oroville Lake levels had decreased by Sunday night as they let water flow from its heavily damaged main spillway.
Mr Croyle said the department will continue releasing as much as 100,000 cubic feet per second from the main spillway to try to reduce the dam's level by 50ft ahead of storms forecast to reach the area on Wednesday.
Department engineer and spokesman Kevin Dossey told the Sacramento Bee the emergency spillway was rated to handle 250,000 cubic feet per second, but it began to show weakness on Sunday after flows peaked at 12,600 cubic feet per second.
The California national guard put out a notification to all of its 23,000 soldiers and airmen to be ready to deploy.
It marked the first time an alert for the entire California national guard had been issued since the 1992 riots in Los Angeles after a jury acquitted four police officers over the beating of Rodney King.
Earlier on Sunday, officials had stressed the Oroville Dam itself was structurally sound.
Unexpected erosion chewed through the main spillway during heavy rain earlier this week, sending chunks of concrete flying and creating a 200ft-long, 30ft-deep hole that continues growing.
Engineers do not know what caused the cave-in.
Chris Orrock, a department of water resources spokesman, said it appears the dam's main spillway has stopped crumbling even though it is being used for water releases.
The lake is a central piece of California's government-run water delivery network, supplying water for agriculture in the Central Valley and residents and businesses in southern California.