After October wildfires in 2003, a Christmas rainstorm sent a boulder-strewn mud flow through a church camp and a campground in the San Bernardino Mountains. Sixteen people died.
Authorities dug through the muck for days, finding all but one of the entombed bodies. Four months later and 15 miles downstream, a Santa Ana River work crew stumbled upon the remains of the last body — a 12-year-old boy from the church camp.
The tragedy was linked to the torched vegetation from the earlier wildfires, which left open forest ground that quickly eroded in the big winter storm, geologists said.
Now geologists worry about a similar threat to people this winter when the rain hits the ground charred by the Rim fire in Stanislaus National Forest and Yosemite National Park.
"The flames may be out, but there's still danger," said geologist Alan Gallegos of the Sierra National Forest, east of Fresno.
Gallegos and fellow Sierra forest geologist Jerome DeGraff are experts in predicting potential debris flows, which are a combination of mud, boulders and tree limbs. Last month, they joined a team of experts studying the aftermath of the 257,135-acre Rim fire.
They say mud flows carrying thousands of tons of debris could cross roads used by lumber company Sierra Pacific Industries, Hetch Hetchy Water and Power and mountain visitors. The Forest Service is warning people in the area to pay attention to weather reports.
"Debris flows can destroy a house or carry off a car," DeGraff said. "Their power is surprising."
A debris flow also could muddy the water quality in the Tuolumne River watershed, which supplies the Modesto and Turlock irrigation districts and San Francisco-owned Hetch Hetchy Water and Power. It serves much of the Bay Area.
Gallegos and DeGraff are scheduled to talk about their debris-flow assessments at the Geological Society of America's 125th anniversary gathering Oct. 27-30 in Denver. The Rim fire will figure prominently into their presentation.
The spectacular blaze, the third-largest in state history, has faded to scattered hot spots after storming through an area one-third the size of Rhode Island. Some describe the worst damage as a moonscape.
But that kind of severely burned acreage amounts to only 7% in the Rim fire, said Stanislaus forest supervisor Susan Skalski.
"That's not a high percentage," she said. "It was a little surprising, considering the intensity of this fire."
About 37% of the fire acreage is considered moderately burned, which also is a factor in debris-flow scenarios in the steep river canyon, Gallegos said.
In these burned areas, the fire created a grease layer of oils from incinerated vegetation, such as chaparral. It's now a slick surface that sheds water, instead of absorbing it as the soil usually would do, the geologists said.
Rain runoff rushes into streams and creeks, picking up speed and building a sudden, murky flow.
"The first storm of real size can start a debris flow," DeGraff said. "You find places that are scoured right down to the bedrock in the stream channel."
As it continues downstream picking up soil, rocks, tree limbs, roots and parts of shrubs, it slowly thickens to the consistency of ketchup, Gallegos said.
"Soon, you've got a river of wet concrete coming down," he said. "It won't stop until it gets to a flatter place where it spreads out and drops the debris, and the water pushes through."
The flow can carry as much as 100,000 cubic yards of debris, enough to fill 10,000 dump trucks.
In the San Bernardino Mountains a decade ago, the powerful debris flow swept away cabins at the church camp, called Saint Sophia Greek Orthodox Campground.
The geologists said such a flow would move quickly through parts of the Tuolumne River watershed, particularly around Cherry Lake and Cottonwood roads.
The roads cross stream channels where the water passes through culverts, which are typically a large pipe that funnels the stream beneath the road.
The geologists recommend closing lightly traveled roads during rainy seasons when there is a possibility of such a flow.
Part of Stump Springs Road near Big Creek in the Sierra National Forest will be closed this rain season because of risky situations created in summer by the Aspen fire, Gallegos said.
But it is impractical to close Cherry Lake and Cottonwood roads, he said. Hetch Hetchy Water and Power, for instance, uses Cherry Lake Road to reach hydroelectric plants in the area.
Gallegos said signs will be posted to warn people about the dangers of driving through the area during a rain storm. Authorities also have spoken to the companies with business in the area.
Gallegos added that crews will shred the remains of some trees to help secure the soil in the Granite Creek area after the Rim fire.
But, more than anything else, the weather will be the determining factor after the last burning embers from the Rim fire die off.
Stanislaus National Forest leaders say barriers will be installed along with seeding for vegetation to prevent erosion. But they understand the challenge.
"If it rains hard," Stanislaus supervisor Skalski said, "we'll have problems."
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