SAN BERNARDINO, Calif. (AP) -- Mudslides often sweep through mountainous regions like those in San Bernardino County after wildfires burn away vegetation that normally would shore up steep terrain and absorb storm runoff.
A blaze burns off layers of twigs, leaves and moss, leaving vast areas of top soil exposed and coated in a waxy resin usually locked in the forest debris.
As a result, the ground is more susceptible to erosion and will retain much less water when the rainy season arrives, sending sheets of storm water racing toward the valleys below.
Those factors were at work Thursday when a drenching winter storm triggered a deadly avalanche of mud and rocks. The mudslides overran a youth camp in Waterman Canyon and a trailer park in Devore, both of which are in the San Bernardino Mountains and about 60 miles east of Los Angeles.
The range was hit hard by deadly Southern California wildfires that burned nearly 750,000 acres last October and November and destroyed more than 3,500 homes. Much of Waterman Canyon was scorched in a fire that consumed 91,000 acres, destroyed nearly 1,000 homes and killed four people.
Many Southern California mountains are particularly susceptible to the fire-flood cycle because their soil is less stable than, for example, the granite-anchored slopes of the Sierra Nevadas.
After the fires, authorities worked to stabilize denuded slopes and identify areas in danger of flooding and mudslides, using straw mulch or berms to retain water and soil and barriers such as sandbags and fallen trees to channel debris into waterways.