Studying ‘car-swallowing’ debris flows to improve hazard assessment in Idaho

That's Robby Jost speaking, a second year masters student in Boise State's Department of Geosciences. He is also alum (B.S, Geosciences, 2015) and is advised by Professor Jen Pierce. The debris flow he described is one of dozens that have occurred since the Wapiti Fire consumed nearly 130,000 acres in 2024. This burned region is his field lab as a graduate student, just as it was for his mentor, Pierce, 20 years earlier.

As pyrogeomorphologists (or someone who studies how fires change the physical properties of the earth and landscape) these debris flows are both a source of scientific intrigue for Pierce and Jost, as well as a poignant reminder that wildfires have lasting impact, long after the last ember is snuffed out.

Members of the Boise State Earth, Wind, and Fire Lab (left to right: Jen Pierce, Ecology, Evolution and Behavior doctoral student Liam Getzloff, and Robby Jost) getting ready to survey post-fire debris flows along Idaho highway 21. Photo provided by Jen Pierce.

"I like to say that geology is the template on which the whole landscape is built, so these debris flows can really create different trajectories for different ecosystems once they come down," Jost said.

Post-fire debris flow sediments have a future…

After each new rainfall in the Wapiti area, Jost prepares for another foray into the rugged mountainsides. His preparation checklist includes:

• Sturdy hiking boots
• Long pants and sleeves to tromp through brush and thick floodplains
• Survey equipment, including either a cell phone or higher precision GPS equipment
• A notebook
• A laser range finder, to gauge distances
• and the humble (but indispensable) shovel

When studying a new debris flow, Jost's first question is about the volume of debris: How much soil, sediment and plant material moved from their original site?

Jost setting up the 'base station' for a high accuracy GNSS (Global Navigation Satellite System, a more general term for GPS) survey. The base station is set up in an area with clear sky view for several hours and relays location data from satellites to a mobile "rover" that Jost carries to map the extent of the post-fire debris flow deposits. Photo provided by Jen Pierce.

He also collects geospatial data about the basins or the catchments where these debris flows came from. This information can give researchers an understanding of how much water and material is moving, as well as the steepness of the basins.

The next question is, how much farther might these particles travel when the next rains come? This question is critical for downstream ecosystems and human infrastructure.

To get a sense of how the debris flow might continue spreading, Jost digs directly into the debris flow and collects one-gallon bags of sediment to take back to the lab at Boise State. He will sift through them and gauge particle sizes and from there, mathematics and physics can paint a picture of how much further sediments of various sizes might be carried.

And they have a past…

For Pierce, Jost's study is a continuation of research that she began in the same area as a doctoral student at the University of New Mexico, from 2000 to 2004. She recalled the many weeks along the South Fork Payette "with my shovel and my sample bags."

In her dissertation, Pierce discovered charcoal from past fires and past fire-related debris flows within sediments along the South Fork Payette River. She sent samples to Lawrence Livermore Labs in California to be carbon dated. The resulting dates revealed more of Idaho's long and cyclical history with fire and debris flows.

Jen Pierce samples sediments from a debris flow in Canyon Creek. Photo provided by Jen Pierce.

"In these same areas that Robby's studying again now, there were very large fire-related debris flows about a thousand years ago during a time called the 'Medieval Climatic Anomaly.' It was a time when it was very warm and dry, though not as warm and dry as it is now. So, history is repeating itself," Pierce said.

However, since the Medieval Climatic Anomaly, world climate trends have changed significantly, and the sediment in the debris flows bears witness.

"What has changed is that it is so much warmer and so much drier now. These forests are burning hotter, they're burning more severely, and they're burning more extensively than they have in at least the last 400 years and probably more like the last 1,000 years," Pierce explained.

Protecting Idaho communities

Jost, Pierce and pyrogeomorphologists across the country share their collection data with the United States Geological Survey. The cumulative information is critical to predicting and preparing communities and land managers for the next debris flow.

An excavator staged in a pullout along Idaho highway 21. Used recently by the Idaho Department of Transportation to clear the road corridor of sediment and wood that was deposited by debris flows on July 22, 2026. Photo provided by Jen Pierce.

These models can then be used to predict outcomes of wildfires, such as the anticipated size of a debris flow, how much water it will take to trigger them, and where they might end up. With that information, land managers and government agencies can plan evacuation routes, road closures and other hazard mitigation methods, in advance.

"That's huge for the post fire response and for government agencies like the Idaho Transportation Department and the National Weather Service," Jost said. "The easiest way to mitigate a hazard is just don't get near it, because risk is partly about exposure."

As a Hailey native and fan of outdoor recreation, Jost has seen his fair share of wildfire hazards - the fires themselves, post-fire river closures and snow avalanches. He said the latter may have been where he first started to become interested in steep terrain and the geological hazards that come with it.

"There's definitely a personal fascination with geologic hazards," Jost said, "but I also really like the communities in the West. I want to keep people aware of hazards and continue living out here. I just want to keep people safe."

Appreciating an ecological paradox

Despite being hazardous to humans, wildfire debris flows must be credited with the critical revitalizing effect they have on Idaho's forested lands. Pierce reminds her students and the public that "Fire and fire-related erosion has always been a factor in our Idaho forests."

"When you first go to a burn area, it looks charred and devastated. But what we see now in the Wapiti burn area are beautiful wildflowers," Pierce said. "We see stands of aspen coming back. Those debris flows have provided the wood and sediment that our fish really need to survive and thrive. So, even though it's a disturbance, not all disturbance is bad."

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