"As firefighters keep battling the blazes, a new menace is emerging: debris flows. The rains are coming to California..."

"... as soon as Tuesday night, and the fires have primed the soil for another disaster that can claim yet more lives.... The problem is a fundamental change to the soil itself. When a wildfire burns vegetation, it releases organic compounds.... This soil normally absorbs water, but the organic compounds turn it hydrophobic, literally meaning 'fear of water.' It is not an understatement: You can pour a bottle of water on this burned soil and it’ll bead right off, as if the dirt were polished marble... Problem number two: vegetation anchors the soil.... The debris flow can carry boulders and trees along with mud, making it more dangerous than a regular flood or a dry landslide.... Boulders can also act like a moving dam, retarding the flow..... If you haven’t cleared out in time and the flow comes, the recommendation is to shelter in place—moving to the upper story of your house, for instance. Out on the streets is exactly where you don't want to be, as the flow follows those paths...."

From "California Fire Survivors Brace for Debris-Filled Mudslides" (Wired).