LOS ANGELES (Reuters) - The death toll from Southern California mudslides that swallowed dozens of homes and forced the closure of a major highway along the picturesque Santa Barbara County coast rose to 20 on Sunday, with four other people still reported missing.

Emergency officials said chances of finding more survivors in the ravaged landscape of hardened muck, boulders and other debris had waned considerably since heavy rains unleashed torrents of mud down hillsides before dawn last Tuesday.

Still, the 20 fatalities confirmed in and around the affluent community of Montecito, 85 miles (137 km) northwest of Los Angeles in the coastal slopes adjacent to Santa Barbara, ranks as the greatest loss of life from a California mudslide in at least 13 years.

The official death toll early on Saturday had stood at 19, with seven people listed as missing. Four remained unaccounted for on Sunday, including the 2-year-old daughter of the latest victim whose remains have been positively identified.

Ten people perished in January 2005 when a hillside saturated by weeks of torrential rains collapsed in the seaside hamlet of La Conchita, just 18 miles (29 km) southeast of Montecito, burying more than a dozen homes in seconds.

Unlike the La Conchita tragedy, the stage was set for Montecito’s slides by a massive wildfire last month -- the largest on record in California -- that stripped hillsides bare of any vegetation to hold soils in place following a day of drenching showers.

Another 900 emergency personnel arrived this weekend to join the relief effort conducted by more than 2,100 personnel from local, state and federal agencies, including the U.S. Coast Guard, the U.S. Navy and the American Red Cross.

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But authorities said on Sunday that the search-and-rescue mission had shifted into a “search-and-recovery” effort, reflecting the diminished likelihood of finding anyone else alive.

The destruction covered 30 square miles (78 square km), leaving 65 single-family homes demolished and more than 450 others damaged. Nearly 30 commercial properties were damaged or destroyed, officials said.

The slides also forced a 10-mile (16-km) stretch of one of California’s most celebrated coastal roads, the heavily traveled Highway 101, to be closed indefinitely.

The shutdown has posed a major traffic disruption, forcing motorists to drive 100 miles out of their way on back roads to commute around the closure, said Jim Shivers, a spokesman for the state transportation department.

He said parts of Highway 101 were under 6 to 7 feet (1.8 to 2.1 meters) of water and mud. Cleanup crews were working around the clock in 12-hour shifts.

Seeking to ease the detour for commuters, ferry boats were making commuter runs twice a day between Santa Barbara and the town of Ventura to the south.

A community group formed in the aftermath of last month’s devastating Thomas Fire also began coordinating free airplane and helicopter rides for doctors and emergency personnel.

As a precaution against the possibility of further slides, officials have ordered residents in most of the southeastern corner of Montecito to leave their homes for what was likely to be one or two weeks.