Rain in the Cascades in northern Oregon brought some relief Friday to crews trying to corral the large wildfires burning there. But the National Weather Service warned that the rain was bringing a new threat: landslides in the steep, burned-out terrain.
The danger was imminent, the Weather Service said, because the heavy rains could create torrents of mud and dead tree limbs that could thunder down the slopes and sweep into communities, with devastating effects.
Josh Roering, a geologist at the University of Oregon, said the region also faced a significant longer-term landslide hazard, beginning three or four years from now, when the roots of burned trees lose their strength to hold the soil in place. Then a heavy rain could saturate and destabilize an entire slope, causing it to slide.
With climate change leading to more extreme rainfall, the risk of this kind of landslide increases.
Dr. Roering said towns along the McKenzie River faced an immediate hazard. The river, which flows into the Willamette Valley, is the site of the Holiday Farm Fire that has burned more than 170,000 acres.
“I think the risk to these communities is substantial,” he said, especially if downpours bring a half-inch of rain or more per hour. “That’s when I start to worry.”
The danger was that one or more areas would be struck by what Dr. Roering called a “California-style” debris flow, like those that occurred north of Los Angeles in January 2018, killing 23 people and causing about $200 million in damage.
These kinds of slides occur when the forest floor is blanketed in waxy compounds from burning trees, so the normally absorbent soil sheds more water. When heavy rains come, the runoff is powerful enough to pick up boulders, tree limbs and other vegetation. It quickly merges into channels as it races down the steep slopes, creating fast-moving rivers of mud and debris. In one of the 2018 California slides, which devastated the town of Montecito, the flow was up to 15 feet thick and moved at about 20 miles an hour.
While such post-fire landslides are relatively common in California, Dr. Roering said, they are actually rare in Oregon. Scientists aren’t sure why that is, but they have a few hypotheses. The topography is roughly similar, but the vegetation is different, and when it burns, it may produce fewer of the compounds that make the soil repel water. Soil in the Cascades is also rockier, which may help it stay more absorbent after a fire.
Debris flows have been studied extensively by the United States Geological Survey in California, where severe fires are far more common than on the western slope of the Cascades. Using data on past flows and computer models, when heavy rain comes after fires the National Weather Service in California issues warnings about the landslide risks in specific locations.
“They have this great empirical understanding of what does and doesn’t create this post-fire debris flow process,” Dr. Roering said. “We don’t have that advantage up here.”
In Oregon, potentially more so than in California, in the next decade there will be a continuing and significant threat from the current fires, Dr. Roering said.
“There is this longer-term kind of hangover that makes all of Western Oregon areas that have burned vulnerable to a different kind of landslide,” he said.
The state already experiences these kinds of landslides from clear-cutting by timber companies.
The effect is the same for clear-cutting or a severe fire: With the trees on a mountainside gone, the roots eventually deteriorate. The soil layer in the Cascades is relatively shallow — shallower than in much of California — and can become saturated in a heavy rain. Without the roots holding it in place, the soil can give way as a discrete mass and slide down the mountain, Dr. Roering said.
The risk is greatest from about 3 to 10 years after the trees are gone, before new growth develops an extensive root system, he said.