Historic wildfires raging from California to Colorado are weakening watersheds and setting the stage for deadly mudslides and flooding and, in some places, threatening to poison critical water supplies.
Fueled by record-setting temperatures and strong winds, blazes are wreaking havoc in the West, decimating entire towns like Malden in eastern Washington state, where 80% of the homes and structures — from the fire station to city hall — were burned to the ground.
But the fires don’t just pose a threat to things that burn. More intense and larger fires are also shifting the very ground in Western states. Severe wildfires can change the hydrologic response of a watershed so quickly that even a relatively modest rainstorm can trigger flash floods and steep terrain debris flows, said Jason Kean, a research hydrologist with the U.S. Geological Survey’s Landslide Hazards Program in Golden, Colo.
“A debris flow is kind of like a flood on steroids,” said Kean. “It’s all bulked up with rocks, mud, boulders, and then it becomes a different animal that can be even more destructive than a flood.”
Burned and denuded land no longer has the vegetative root structure to help stabilize the soil and is easily eroded by rain, Kean said, adding that the land also can’t absorb water the way it did before the fire. High-severity fires can also cause soil surfaces to harden or even cause soil to repel water, Kean said.
It’s a scenario that’s played out more and more as the size and intensity of fires grow in a warming world.
In December of 2017, one of the largest fires in California — the massive Thomas wildfire — burned through large swaths of Santa Barbara and Ventura counties. In January, before the fire was contained, intense bursts of rain fell on a portion of the burn area above the city of Montecito, Calif., weakening the watershed there. A massive slew of boulders and debris overwhelmed the town without warning, killing 23 people and destroying more than 400 homes in its path.
Another lesser-known threat to the region’s water is gaining attention in urban areas affected by wildfires: chemical contamination.
In cities that have experienced devastating fires, water officials are finding cancer-causing benzene and other volatile organic compounds in contaminated and fire-damaged water infrastructure.
Such was the case in the Northern California town of Santa Rosa after the Tubbs Fire in the fall of 2017 and again in the town of Paradise after the deadly 2018 Camp Fire. Earlier this week, the carcinogen was detected in the Riverside Grove neighborhood near Boulder Creek, Calif., a community devastated by the CZU August Lightning Complex fires.
Andrew Whelton, an associate professor of civil, environmental and ecological engineering at Purdue University, said such contamination is gaining more attention — and scrutiny — given the lack of water testing after wildfires.
E&E News, 11 September 2020