In August of 2020, the Grizzly Creek fire burned more than 30,000 acres around Interstate 70 in Colorado's Glenwood Canyon. The following summer, several rainstorms swept across the area and sent devastating debris flows, fast-moving torrents of mud, rocks, and water coursing across the highway. The road was closed for weeks, causing an estimated economic cost to the national economy of about $1,000,000 every hour*.
Two years after the fire, officials were braced for more debris flows to be triggered by even light rains. They shut down the interstate several times as a precaution, warning that the closures would continue until geologists determined that there was no longer a risk. However, in 2022, the debris flows never happened, as vegetation recovered and restabilized the hillslopes.
"The last thing we want is for vital safety warnings to be seen as 'crying wolf' and ignored by people," says Andrew Graber , a landslide hazards research geologist at the U.S. Geological Survey (USGS), "so we set out to discover what storm intensity is actually needed to trigger debris flows in postfire landscapes where the trees and plants have begun to recover. And we have made great progress towards that answer." Their findings are published in a recent issue of Geosphere .
Graber and a team of scientists at the USGS developed a new method for predicting debris flows that incorporates vegetation recovery in the years after a fire. It appears to reduce the number of unnecessary warnings while not increasing how many landslides the method missed.
After a wildfire in a steep landscape, multiple factors contribute to an increased debris flow hazard. These include changes to the soil itself that prevent it from absorbing water and the absence of plants that take up water and reduce the amount of runoff. USGS scientists routinely provide postfire debris-flow hazard maps and rainfall threshold guidance to national and state agencies, like the Colorado Department of Transportation, who then make decisions about whether to issue warnings or close highways when storms approach.
Before his team's update, "those maps and those hazard predictions would be capturing the worst-case scenario right after fire," says Graber.
To improve the maps and hazard predictions, especially after plants start to regrow, Graber and the team updated their equations and incorporated satellite imagery that measured the degree of vegetation recovery. They then went back over three years of rainfall data in 12 burned areas and compared their predictions to the actual number of debris flows that occurred.
"Once we compared our updated postfire forecasts to real debris-flow data, we were glad to see how much better they reflected what actually happened after the fire," says Graber.
Better warnings are especially important as more people move into steep, fire-prone landscapes in the American West. Compounding the hazard, climate change contributes to more wildfires, droughts that might delay vegetation recovery, and more intense storms that could trigger debris flows. But, Graber cautions, the warning system is still not perfect, and scientists are still working on improvements to generate even more accurate warnings.
"No forecast perfectly predicts when, where, and how big a debris flow might be," says Graber. "But our recent improvements bring us an important step closer and should help alleviate some of the concerns with too many warnings being issued."
*Denver Post: Rain triggering rock, mudslides forces I-70 shutdowns in Colorado as new safety norm , 22 August 2022
CITATION: Graber, A., et al., 2026, Characterizing changes in postfire debris-flow hazard as burned areas recover : Geosphere, https://doi.org/10.1130/GES02936.1
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The Geological Society of America (GSA) is a global professional society with more than 18,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked "geology" journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon's top 100 best-selling geology titles.
