The death toll in the mudslides that buried parts of Montecito, Calif., in muck and boulders has risen to 20, officials said on Sunday, and portions of Highway 101 will remain closed indefinitely.

“No one could have planned for the size and scope of what a 200-year storm, immediately following our largest wildfire, would bring,” Eric Peterson, the chief of the Santa Barbara County Fire Department, said at a news conference on Saturday.

Road crews are working 12-hour shifts to reopen the highways, said Jim Shivers, a spokesman for the California Department of Transportation, known as Caltrans.

Highway 101, a major artery on the Central Coast used by up to 79,000 vehicles per day, is closed for a stretch of about seven miles from Carpinteria to Santa Barbara, Mr. Shivers said in an email on Sunday.