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Officials in Santa Barbara County, California, have warned that they need to clear debris from drainage basins and channels after last week's deadly mudslides or risk possible further mud flows should another storm hit Southern California.

The warning came as authorities scaled back their efforts to find survivors amid what California Highway Patrol Capt. Cindy Ponce described Sunday as the "heart-wrenching" aftermath of Tuesday's mudslides, which has killed at least 20 people, including four children ages 3, 6, 10 and 12.

The mudslides occurred after a heavy downpour and struck areas burned by the so-called Thomas Fire, which scorched hillsides last month.

Sheriff Bill Brown said at a news conference that the decision to move from a search-and-rescue to a search-and-recovery mission was a difficult one and was made only after extensive searches that reached all way to the Pacific Ocean.

"It is more than 130 hours since this tragedy occurred," Brown said. "That necessitates the need to transition."

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Four people were still missing, including 2-year-old Lydia Sutthithepa. Earlier Sunday, the sheriff's office said Lydia's father, Pinit Sutthithepa, 30 was found dead on Saturday. Earlier in the week, the bodies of Pinit's 79-year-old father-in-law and 6-year-old son were also found.

Pinit Sutthithepa, who immigrated to the United States from Thailand, worked at a Toyota dealership and made enough to bring his wife and two children to California in the summer of 2016, a friend, Poy Sayavongs, told the Santa Maria Times.

"It's cruel — they only had a short time together before this tragedy struck," Sayavongs told the newspaper.

The mudslides clogged drainage basins and channels with debris, officials said. A section of Highway 101 — a main north-south artery through California, Oregon and Washington — also remains closed indefinitely because of the high levels of water on the road surface, the state Transportation Department said.

Mudflow and damage from mudslides in a photo taken from a Santa Barbara County Air Support Unit Fire Copter over Montecito, California, on Wednesday. Matt Udkow / Santa Barbara County Fire Department via Reuters

Rob Lewin, director of the Santa Barbara County Emergency Management Office, said Saturday that the blocked drainage channels in the Montecito area of the county meant that even if a storm of less intensity hit, the area could see "more mud and flow."

"We have got to get those basins cleared as fast as we can," Lewin said. "If we don't get those debris basins cleared out, then we're not going to be prepared for the next storm, and we don't know what that storm is going to look like."

Lewin said clearing the debris was his team's No. 1 priority to mitigate the risk of another mudslide. He said a task force was working to find places to safely move the debris.

"We have got to get those channels ready or we're going to have more mud and flow under a storm that would be of less intensity," he said.

The seven-day forecast for the Montecito area has no storm warnings and is largely dry with a small chance for light rain on Friday, according to The Weather Channel.

As for Highway 101, Jim Shivers of state Transportation Department said Sunday that clean-up efforts had made "sure and steady progress."

The water had begun to recede, he said, and nearly 100 yards of debris had been dug up from the highway's southbound lanes. On the other side of median, more than 150 yards had been removed.

"Within the next 24 hours, we hope to some guidance or schedule for reopening U.S. 101," he said.