Thousands of Southern Californians fled their homes on Monday as a powerful rain storm that could cause flash floods and trigger mudslides soaked steep slopes where a series of intense wildfires burned off vegetation last month.
Officials did not provide any further information on the girl's condition or if any of her family members had been trapped.
Some 500 firefighters and other rescue workers were searching debris spread across a wide area of Montecito, a wealthy area of about 9,000 people northwest of Los Angeles.
With roads blocked, rescue crews used helicopters to pluck people from their rooftops.
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Authorities said more than 100 homes had been completely destroyed and another 300 damaged.
"We realise that this is going to be a long and hard journey for all of us and our community", Santa Barbara County Sheriff Bill Brown said.
A short drive from the centre of Santa Barbara, the small neighbourhood of Montecito, known as an enclave of celebrities and billionaires, was one of the areas worst hit by the mudslides.
"It looked like a World War I battlefield".
Several roads are closed due to mudslides and debris, including the major thoroughfare Highway 101.
A month ago, Ventura and Santa Barbara counties battled the Thomas Fire, the state's largest wildfire on record, which - fanned by strong winds and dry brush - burned 281,620 acres. Among the homeowners affected by the storm was the actor Jeff Bridges, who said on Twitter that his home was "severely damaged".
Winfrey said she was "praying for our community again in Santa Barbara" as she posted a video on Instagram showing the depth of mud outside her home.