Nearly 200,000 Northern Californians who live downstream of the country's tallest dam were allowed to return home on Tuesday after two nights of uncertainty, but they were warned they may have to again flee to higher ground on a moment's notice if hastily made repairs to the battered structure do not hold.

The fixes could be put to their first test later this week with the first of a series of small storms forecast for the region.

But the real test is still to come in the weeks ahead when a record amount of snowfall melts in nearby mountains.

"There is the prospect that we could issue another evacuation order if the situation changes and the risk increases," Butte County Sheriff Kory Honea said Tuesday, telling residents they could return home but to remain vigilant.

Residents living below the Oroville Dam were suddenly ordered to evacuate Sunday afternoon after water authorities had assured them for nearly a week that the dam was sound despite a gaping and growing hole found in the structure's main spillway.

The order came after authorities feared an earthen emergency spillway, used when the lake behind the dam overflows its capacity, appeared ready to fail Sunday because of erosion.

Over the weekend, the swollen lake spilled down the unpaved emergency spillway for nearly 40 hours, leaving it badly eroded.

Government failures

The problem occurred six days after engineers discovered a growing hole in the dam's main concrete spillway.

State and federal officials ignored calls in 2005 from environmental groups to armour the earthen spillway in concrete to prevent erosion.

Federal regulators concluded the earthen spillway could handle a large amount of overflow after water agencies, which would have had to pay for the upgrade, argued it was unnecessary.

On Tuesday, John Garamendi, a Democrat who represents an area near Oroville, called the government's failure to coat the spillway in concrete "a classic case of woulda, coulda, shoulda".

He said that if the state had listened to the 2005 warnings and installed the concrete a decade ago, "this problem would not have occurred. But they didn't, and there are probably multiple reasons why", with cost a crucial one.

The California Department of Water Resources said the lake was ready to take on rain and melting snow. State water officials said they have drained enough of the lake behind Oroville Dam that the emergency spillway will not be needed to handle runoff from an approaching storm.

Forecasts call for five to 10cm of rain and snow in the foothills and mountains starting Wednesday night. But the storm was looking colder than initially projected, meaning less rain and less runoff than last week's storms.

Acting department chief Bill Croyle said 40 trucks have dumped 30 tonnes of bags loaded with sand, concrete blocks and boulders every hour into the damaged areas, while helicopters have dropped bags of rocks and cement blocks onto the problem sites.

The damaged main spillway has been stable for four days and handling a heavy flow of water, reducing the reservoir's water level by 4.5 metres in preparation for coming rain and melting snow throughout the spring. Officials hope to drain the lake another 11 metres.

"We still have a large snowpack; we will see quite a spring runoff," Croyle said.

Croyle said the goal is to keep the reservoir below capacity so the use of the auxiliary spillway would not be used. Still, Croyle said that spillway has been repaired and that he is confident it could be used again if needed.

Preliminary estimates say permanently fixing the hole in the main spillway could cost $100 million to $200 million, Croyle said.

READ MORE: Damaged California dam prompts urgent evacuations

Experts are drawing up plans for repairs that will begin after the spring runoff season ends.

But after two days away from home, tens of thousands of evacuees were growing weary. They welcomed the news that they could return home.

"You don't appreciate home until it's taken away from you," said Margaret Johnston, 69, of Oroville, who spent two nights at a church with her two sons. She had packed a few blankets, pillows and clothes into a black rubbish bag.

Returning residents vowed to heed the sheriff's warning to remain vigilant.

Rod Remocal said he and his wife would now be ready to leave their Biggs home near the dam at a moment's notice after fleeing in a rush Sunday.

"We're all coming back and pack and be ready this time," Remocal said. "This time, we're going to be on call like they said."