Last summer¹s wildfires were bad. This summer¹s debris flows could be worse.

MARILYN WEISS/AP

Car 54, Where are you? Sun Valley, California

"I'VE BEEN TO A HOUSE where it came in the back door and out the front door," says Bill Dietrich, Director of Earth and Planetary Sciences at the University of California, Berkeley. "It looked like a crazy painter went through." He's talking about a Malibu-area bungalow that survived a close call with a 1993 brush fire, only to end up awash in rocks and mud when the winter rains arrived months later.

Common sense dictates that the worst part of a forest fire is, well, the fire. But geomorphologists (scientists who study erosion on the earth¹s surface) will tell you intense infernos like those that torched over 5 million acres of the West last summer often do the most damage once the flames die down and the rains come. The resulting Slurpee of soil, rocks, and water washed off a scorched hillside—the "debris flow," in geomorphologist parlance—can pour down a slope at up to 25 miles per hour, smashing homes, clogging streams, and carving entirely new landscapes along the way. With acres of steep mountainous terrain across Colorado, New Mexico, Idaho, and Montana no longer held together by the roots of plants and trees, University of New Mexico geologist Grant Meyer says it could take only a few severe storms to deliver a century's worth of erosion. A year after the 1988 Yellowstone fires, a storm triggered debris flows in Gibbon Canyon, burying a camper trailer seconds after a family of three escaped. In 1995, two months after a fire on Storm King Mountain, debris flows swamped three miles of I-70 in Colorado, burying 30 cars and pushing some—along with their occupants—into the Colorado River (no one died).

This summer, scientists worry that heavy rains across the scorched Rockies could set off hundreds of small flows, and a few big slides. But, as Dietrich points out, it¹s all part of the normal geologic ebb and flow. "The world erodes," he says. "That's why we have hill slopes." —Jason Paur