Nearly 200,000 people who were ordered to leave their homes after a damaged California spillway threatened to unleash a nine-metre wall of water may not be able to return until significant erosion is repaired, authorities said Monday.

The officials who issued the hasty evacuation order defended their decision, saying it was necessary to ensure public safety in the region downstream from the nation's tallest dam, about 240 kilometres northeast of San Francisco.

Engineers spotted a hole in the spillway, which they feared could have failed within an hour.

The water level of the massive reservoir known as Lake Oroville dropped Monday, slightly easing fears of a catastrophic collapse. But with more storms on the horizon, crews raced to assess what happened and began dumping large boulders and sandbags into the spillway to prevent any more erosion.

The acting head of the state's Department of Water Resources said he did not know if anything had gone wrong and was unaware of a 2005 report that recommended fortifying the earthen emergency spillway with concrete for just such an event. The spillway had never been used in the dam's nearly 50 years of operation, and it was not near capacity when it began to fail.

"I'm not sure anything went wrong," Bill Croyle said. "This was a new, never-having-happened-before event."

Croyle and the local sheriff sought to reassure the public that downstream communities were safe until water began spilling over the lake's edge Sunday and a chocolate-colored torrent of water began chewing through the slope below it.

Water flows over the spillway of the Oroville Dam in Oroville, Calif., on Friday. (Josh F.W. Cook/Office of Assemblyman Brian Dahle/Associated Press)

The Department of Water Resources said conditions were stable at noon Sunday and then tweeted an evacuation order at 4:45 p.m., warning of a possible failure within the hour, saying "this is not a drill."

Chaos ensued as anxious residents rushed to pack up their families and abandon several communities in Butte, Yuba and Sutter counties. The mostly flat northern Central Valley is known mainly for its sprawling agriculture industry, which is fed by dammed-up rivers that spill down from the Sierra Nevada foothills nearby.

'People were just panicking'

It took some people seven hours to travel to evacuation centres that should have been an hour away, said Chico Councilman Andrew Coolidge, who visited with evacuees in packed shelters in his city.

Butte County Sheriff Kory Honea insisted the evacuation had been accomplished in a "fairly timely fashion" and a "fairly orderly manner."

Patrick Miner, of Live Oak, Calif., said he and his family in a caravan of four cars returned home after driving back roads for two hours and feared they would run out of gas. They didn't bother trying to fuel up because lines were hours long at gas pumps.

"People were just panicking, honking and yelling at each other," he said. "I just got nervous and decided to hunker down. It was pretty scary."

Tens of thousands told to evacuate due to flood danger 0:43

Honea could not estimate when people will be able to return home, but said he's working on a "repopulation plan" that would let residents return to areas least at risk of flooding.

The threat emerged after weeks of storms dumped rain and snow across California, particularly in northern parts of the state.

While the wet winter has been a boon for water supplies, it has created a challenge for water managers who must balance the need for water and hydroelectric power with protecting flatland communities from floods during times of excessive rain or rapidly melting snow.

The Department of Water Resources began releasing water down its main spillway last week to make more room in the reservoir behind the 235-metre-tall Oroville Dam.

Crews are working to repair and reinforce the dam before Wednesday when more rainfall is expected. (Jim Urquhart/Reuters)

After a chunk of concrete tore out of the spillway — creating a hole that is 60 metres long and nine metres deep, and continues to grow — water managers began using the emergency spillway for the first time in its 48-year history. Eventually, the flow of water ripped through a road below and carved out deep chasms in the ground.

Workers used helicopters and backhoes to dump large boulders and sandbags in place on Monday as the main concrete spillway was used to drain water from the lake at about three times the rate it was filling up. Officials want the level to drop 15 metres to make room for storms expected later this week.

Stability concerns previously raised

Environmental groups raised concerns years ago about the stability of the emergency spillway, but state and federal officials dismissed them and insisted the structure was safe, according to records.

In 2005, three advocacy groups complained to the Federal Energy Regulatory Commission that using Lake Oroville's earthen spillway would cause significant erosion because it was not armoured with concrete.

They said soil, rocks and debris could be swept into the Feather River, potentially damaging highway bridges and power plants. The groups warned of a complete failure of the dam itself, threatening lives and property.

(Scott Galley/CBC)

Nearly three years later, state officials said no "significant concerns" about the emergency spillway's integrity had been raised in any government or independent review.

The Department of Water Resources estimated repair costs last week at $100 million US. Officials refused to update that figure Monday.

Gov. Jerry Brown issued an emergency order Sunday directing all necessary resources to deal with the problem, but he remained out of the spotlight Monday.

U.S. Senators Dianne Feinstein and Kamala Harris, both Democrats, called on President Donald Trump to provide $162.3 million US in disaster assistance requested by California.