If you stopped On the banks of Colorado’s Cache la Poudre River after the 2020 Cameron Peak fire, the rumbling water may have appeared black. This mixture of ash and charred earth cascaded into reservoirs that supply drinking water to the downstream city of Fort Collins, home to about 170,000 people. Though the water appeared clear again several weeks later, Charles Rhoades, a research biogeochemist at the U.S. Forest Service’s Rocky Mountain Research Station, says he still sees contaminants from the fire in the watershed.
Recent studies have found that while some watersheds start to recover In the five years following a fire, others can be fundamentally altered and never return to pre-existing conditions. And with wildfires becoming more common, much larger and burning longer as the world warms, hydrologists, ecologists and water management officials are scrambling to understand and mitigate the consequences that fire-polluted water can have on humans and ecosystems.
In a healthy forest, there is a lot of “trash” on the ground: pine needles, dead leaves, debris. “They act like a sponge,” says Rhoades. “As rain falls, it slowly moves through that layer and can seep into the soil.” When fires burn land, they burn off that vegetation and organic matter, leaving behind a bare landscape that is highly susceptible to erosion. Instead of seeping into the soil, rain will slide right off the surface, moving quickly, kicking up soil and carrying it into streams and rivers. Not only does this cause sediment buildup, it can alter water chemistry. Rhoades found that high levels of nutrientssuch as nitrogen, in rivers nearly 15 years after a major fire. These nutrients can cause harmful algal blooms, although they do not directly affect drinking water quality. But other sites show higher levels of heavy metals such as manganese, iron and even lead after a major fire, which can complicate water treatment processes.
Other regions in the western US, such as Taos, New Mexicoand Santa Cruz, Californiahave faced similar problems, as wildfires increase in frequency and duration due to climate change and decades of fire suppression practices. For much of the 20th century, the U.S. Forest Service and other land management agencies tried to prevent fires from occurring, believing it was the best way to protect forests. But natural, low-severity fires improve forest health, preventing the buildup of dense underbrush and dead trees that act as fuel.
“We have a huge buildup of fuel on the landscape from 140 years of fire suppression, and we know that the consequences of that, combined with increasing extreme weather, make the likelihood of really intense fire behavior much higher than it used to be,” says Alissa Cordner, an environmental sociologist and professor at Whitman College in Washington state and a volunteer wildland firefighter. “We also have more and more people living near forests and migrating to places at the interface between forests and urban areas.” Any municipality is at risk of water pollution if a wildfire sweeps through its watershed.
“Consumers rarely know everything that’s going on under the hood,” Rhoades says. After a wildfire, water providers work tirelessly to ensure residents aren’t impacted at their taps, which requires collaboration between territorial agencies, such as the Forest Service, the U.S. Geological Survey (USGS), and local government bodies. They conduct regular water testing, install sediment control structures, and sometimes modify water treatment protocols to deal with the increased contaminant load.
