"... It could get downright biblical.
... When the fires burn hot, as they did in the recent weeks, the plants of the chaparral -- the chamise, scrub oak and California lilac -- release a compound that mixes with the topsoil and makes the earth literally waterproof.
This residue, said Harp of the USGS, "you can't see it, you can't taste it. It has no color and it doesn't degrade in sunlight. You can toss a handful of dirt into a bucket of water and it doesn't get wet. It floats. You'd be amazed."
They call this "non-wettable soil," Harp said. "I had some of this soil in the lab, in water for three or four months, and it never got wet."
When rain -- and in some cases even a heavy mist -- falls upon a hillside that has been charred, its plants and roots burned and gone, and nothing left but water-repellant dirt, there is great potential for avalanches, debris flows, mud floods and other "mass movements."
The top six inches of the slope can literally float away, carrying with it boulders the size of Volkswagens ...