About 2,500 people were stranded after thunderstorms caused mountain mudslides in Southern California, while one person was found dead in a flooded creek, authorities said.

A body was found Sunday in a car that was swept into the rain-swollen water course in Mount Baldy and overturned, San Bernardino County Fire spokesman Chris Prater said. Local media identified him as Joo Hwan Lee, 48, of El Segundo, Calif.

Further east, flash floods Sunday brought thick debris flows that cut off access to two towns. About 1,500 residents of Oak Glen, and another 1,000 residents of Forest Falls in the San Bernardino Mountains were unable to get out because the roads were covered with mud, rock and debris, authorities said.

The stranded include 500 children and adults who had arrived at a Forest Falls campground Sunday morning.

"Our concern is that they're isolated at that campground and no longer have access out of the mountain," Kyle Hauducoeur, another county fire spokesman, said.

Officials said the American Red Cross was responding on Monday with water and food.

Authorities made reverse 911 calls to urge residents to stay put while crews clear the roads with bulldozers — a process that could take up to three days before traffic can move freely again, Hauducoeur said. The muck was so thick it submerged a van in Forest Falls, while on Mount Baldy water swept a hot tub into the road.

Flash floods led to the rescue of several people. Hauducoeur said a woman in Mt. Baldy was rescued from her house before it was buried in mud. Four additional homes in the Bear Creek area were damaged by the debris flow, he said.

'Sounded like a freight train coming through'

San Bernardino County resources were stretched thin by the storm. Scores of swift-water rescue teams and fire engines had been dispatched to far-flung areas, county Fire Capt. Josh Wilkins told the Los Angeles Times.

"Every rescue unit we have, every fire engine we have in San Bernardino County" had been sent out, Wilkins said. "We are literally approaching the maximum right now in terms of our call volume."

In the Mt. Baldy area, debris flows were heading toward homes. Creeks swelled into rivers, submerging cars, Wilkins said. Authorities issued an order telling residents to shelter in place. One group rescued by emergency crews had been trapped in a home that was threatened by flooding and moving debris.

In the Angeles National Forest, a group of four or five people and a dog were airlifted to safety.

A U.S. Forest Service spokesman told KNBC-TV some campers had seconds to evacuate before a torrent of water washed their tents and belongings.

"It sounded like a freight train coming through," Robert Ethridge said.

Monsoonal moisture brought brief but fierce storms to mountain, desert and inland areas. In and around Palm Springs, knee-deep water flooded city streets and stranded vehicles. In the city of Redlands, the storm downed a tree and knocked out power to a few neighbourhoods.

The downpour dumped as much as nine centimetres of rain on Forest Falls, and nearly 13 centimetres of rain on Mount Baldy, the U.S. National Weather Service said.

Authorities said crews were assessing the extent of damages caused by the storms.