Nearly 200,000 Californians faced an indefinite stay in shelters on Tuesday, as engineers worked around the clock to fix the United States' tallest dam before more rain arrives.

After what looks to be the wettest winter in Northern California following years of drought, more rain was forecast for as early as Wednesday and will continue until Sunday, according to the National Weather Service.

Hundreds of families were camped out in cars and tents at Silver Dollar Fairgrounds, in Chico, about 40km northwest of Oroville.

"I left everything in my house. I've got a four-bedroom, two-bathroom house and I don't know what's going to happen to it," said William Rigsbay, 53, of Thermalito.

Crews were working to shore up an emergency overflow channel at the Lake Oroville Dam, using helicopters and heavy construction equipment to place rocks and other materials into areas of erosion, the California Department of Water Resources said.

The agency said the reservoir was draining at a rate that should allow it to absorb inflows from forecasted rain.

The repair work was being done on the back-up channel, known as the emergency spillway, which allows water out of the reservoir when capacity is reached. The primary spillway is also damaged, but it is still usable, officials said.

Water authorities had been relieving pressure on the dam through the concrete-lined primary spillway last week, but lake levels rose as storm water surged in and engineers moderated use of the damaged primary spillway. The rising water topped over the earthen back-up spillway, which has a concrete top, for the first time in the dam's 50-year history over the weekend.

When the emergency spillway showed signs of erosion, engineers feared a 9 metre-high section could fail, and about 188,000 people were ordered to leave their homes in the Feather River valley below the dam, 105km north of Sacramento. Both spillways are next to the dam, which itself is sound, engineers say.

As of Tuesday, officials had yet to indicate when it would be safe for people to go home.

Shopfronts and malls were closed and traffic was light on California's state highway 99 near Oroville. The packed car park of a 7-Eleven store in nearby Live Oak was one of the few signs of life along the route, other than emergency personnel.

Authorities said they had averted the immediate danger of a catastrophic failure at the dam that could unleash a wall of water three stories high on towns below.

"We're doing everything we can to get this dam in shape that they can return and they can live safely without fear. It's very difficult," California Governor Jerry Brown told a news conference on Monday evening.

That day, Brown had sent a letter to US President Donald Trump asking him to issue an emergency declaration that would open up federal assistance for the affected communities.

White House spokesman Sean Spicer on Tuesday told reporters that the administration would "make sure we are doing everything we can to attend to this matter" and "help people who have been impacted", adding that the dam was evidence that the US needed to overhaul its infrastructure, one of Trump's domestic goals.

In the meantime, people living beneath the dam were homeless.

Yolanda Davila, 62, of Thermalito, ended up at the Silver Dollar Fairgrounds, one of only five places in the area taking people with pets. She left home without medicine and dog food in the rush to find shelter before the evacuation deadline.

She said areas such as Sacramento had been issued with flood warnings earlier in the week and that authorities should have warned residents near Oroville much sooner.

"We didn't have a plan. All we knew is to head north towards Chico," Davila said. "If I knew we had to get out earlier I would have went to the Bay area."

The earth-filled dam is just upstream and east of Oroville, a town of about 16,000 people.