If you are beside a river and the water suddenly turns muddy, leave immediately. It could mean that a debris flow is on its way toward you, and it could be catastrophic.
This past summer, like several before, brought a grim reality of wildfires around the globe. Oregon was no exception. In our state, over one million acres burned during the 2020 wildfire season. By end of the fall of 2020, we have already felt the devastation and suffered loss, yet the fires aftermath forecasts more peril – landslides and heavy runoffs.
Wildfires strip the vegetation, leaving the forest floor covered in waxy compounds from burning trees. This coating impairs soil’s ability to absorb rainwater. Heavy rain runoffs merge into channels and subsequently form rivers which rush down the slopes picking up boulders and limbs. In California, these common mud rivers carrying debris, reach heights up to 15 feet and speeds approaching 20 miles per hour. This phenomenon has been studied by the United States Geological Survey.
This scenario has also happened in Oregon – previously due to clear cutting, but now due to massive wildfires.
Josh Roering, a geologist at the University of Oregon, explained in an interview with the New York Times, “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.” The worry starts, according to Roering, with even a half-inch per hour rainfall, “the risk is greatest from about 3 to 10 years after the trees are gone, before new growth develops an extensive root system.”
One of the places of concern is the McKenzie River Corridor, ravaged by the Holiday Farm Fire this year. Althea Rizzo, geologic hazards program coordinator for the Oregon Office of Emergency Management, advises a heighten awareness of creeks and rivers – their sudden muddy appearance might be a clue.
“We don’t have an alarm system for these types of debris flows,” Rizzo tells Oregon Public Broadcasting. “They’re frequently preceded by cracking branches and loud pops. Those root systems are starting to break and crack. Make sure that you have a grab and go bag, ready to go in case something happens.”
Rizzo also warns against driving or walking through flooded areas, even a few inches of flowing water can knock a person down or carry off a vehicle.
“So,” Rizzo says, “it’s just another example of why Oregonians really need to be prepared for any natural or manmade disasters.”
By Joanna Rosinska
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