The death toll from mudslides that devastated parts of California's scenic Santa Barbara County has risen to 20, amid a massive influx of emergency crews searching for four people still missing.

One missing person was found alive yesterday but chances are dwindling fast that more survivors could still be located from the torrent of mud and debris that struck on Tuesday, said Santa Barbara County Sheriff Bill Brown.

"While every hour it remains less likely we'll find anyone alive, there remains hope," he said at a news conference.

Mr Brown named the 19th victim as Morgan Corey, 25. The body of her 12-year-old sister Sawyer had previously been found after their home was swept away this week.

Seven people had been missing at the start of the day, which saw about another 900 emergency personnel arrive in Montecito, north of Los Angeles, to join the relief effort under way by more than 2,100 personnel from local, state and federal agencies, including the US Coast Guard, the US Navy and the American Red Cross.

The ramped-up rescue effort is in response to urgent requests for additional manpower made earlier in the week.

"We need that number to effectively meet our objectives," said a spokesperson for the multi-agency response team.

"To get people here takes time and we're finally getting that request for influx."

On Friday, the sheriff's office made a plea for information on any of the missing residents, who range in age from 2 to 62.

The disaster struck on Tuesday after heavy rains soaked the area near Montecito, where vegetation had been denuded by the largest wildfire in California's history.

Sodden hillsides gave way, unleashing a torrent of mud, water, uprooted trees and boulders onto the valley below and killing victims aged from 3 to 89.

The destruction covered 78sq/km, the California Department of Forestry and Fire Protection said.