Wildfires carry a lot of obvious risks: burning houses, fencelines and pasture. But what happens in the first few years after a fire has its own hazards, too.
A big one is landslides, also called debris flows. That happened outside of Dayton last summer, in the burn scar of the Elk Fire. Jaime Kostelnik is a geologist in the U.S. Geological Survey’s Landslide Hazards Program based in Golden, Colorado. She and a team studied this area specifically. They recently published an interactive story map, with photos, visualized datasets and a chronology of the slides’ impacts. She spoke with Wyoming Public Radio’s Nicky Ouellet.
Editor’s Note: This conversation has been lightly edited for clarity and brevity.
Jaime Kostelnik: Basically, we say a debris flow is a flash flood on steroids. They kind of start moving down slope, and then the flow picks up some trees and it picks up some boulders, and it gets higher and higher. It carries a lot of material. We say it “bulks up.”
They can travel as fast as 35 miles an hour, and they can travel miles beyond the burn area, depending on the rainfall conditions [and] the topography where they initiate. It's like a slurry of concrete, a slug of concrete moving downhill very, very quickly, not slowly. They can and do take out houses and cars and really anything in their path.
NO: On July 15, 2025, thunderstorms hit the area where the Elk Fire had burned a little less than a year before. This was a 100,000-acre fire in the fall of 2024 on the east side of the Bighorn Mountains. I think you visited the site shortly after this storm hit. Can you paint me a picture of what you saw?
JK: Prior to that storm, [the] Landslide Hazards Program had installed a monitoring station in one of the drainages in the Elk Fire. That monitoring station just collects rainfall data. It has a video camera that is triggered when it rains at a certain intensity. It measures soil moisture conditions.
We had prepared that something could happen there because it was steep and our hazard map showed that there was a high likelihood of hazard and there was a highway, Highway 14, was just down slope of that burned drainage.
Folks on our project went out, I did not go on the first visit. They were out there immediately a week, eight days later. They saw where the [Wyoming] Department of Transportation had cleaned up on Highway 14. We also follow the media. We saw that guardrails had been knocked out. The road had been covered with mud, rocks, other debris.
Then we just hike that whole burn area and we walk through and we say, ‘Okay, something happened at this road, but where in the back country did things happen? Can we go out and see that channels or stream beds that had been empty prior to the storm were now filled with rocks and burned vegetation, mud?’
Then we take a little GPS unit out and we click, ‘Yes, this happened here. No, this didn't happen here.’ That data is what's used to support research.
NO: These rainfall events that trigger these slides or debris flows, how much rain are we talking about?
JK: Rainfall rates are not fun and interesting, but we look at 24 millimeters an hour. That's when your windshield wipers are on high. So when you're driving and you have to go from that low setting to a high setting, that's enough to trigger one of these things.
It did rain harder than that in July. It rained just under a half an inch for 15 minutes, [which] is what kicked those flows off. The harder it rains, the bigger these things can be.
NO: Why are areas that have previously burned more likely to slide than other areas?
JK: Because you have removed the vegetation. [Normally] you're intercepting rainfall by trees and shrubs and grass, so that's no longer happening. When the soil reaches a certain temperature, it changes the properties and it becomes hydrophobic, which basically [means] it's repelling water rather than absorbing the rainwater the way it would before.
NO: This is like when I neglect my house plants and then [water] just runs right through.
JK: Right, exactly. If you look out the window and it's raining on the grass, it's one thing. Then you look on the road and all of a sudden the road has turned into a river. So imagine a burned mountain is behaving as if rain is falling on the road surface rather than a surface that's covered with grass, other vegetation.
NO: What are some of the other impacts that these flows can have?
JK: The joke is: If a debris flow happens in the forest and no one's around, does anybody care? Other than the seven of us that work on this project.
But it turns out that there are trails and campgrounds that will often remain closed even after an area starts to recover, because there is still an elevated risk of flash flooding and debris flows.
The water quality, if it's the water source for a community, then that's a real problem.
They have moved fish populations in high hazard areas because it is a certain type of cutthroat trout stream, and if this sediment comes down and clogs up this particular tributary, then it could result in a large fish kill.
Those are all impacts that we've seen just in the last several years.
NO: How long does that heightened hazard risk remain?
JK: As a rule, it remains elevated one to three years. So that depends [on] the recovery, as we call it. Also depends on [if] you're not getting rain, so vegetation is not coming back.
I will say when we were out there [Elk Fire area] in September, sure, it had greened up. These areas come back sometimes faster than people expect, when it's so devastating to see this beautiful landscape that's been charred. But one to three years is really the window.
NO: Is every fire – are the conditions right for these kind of debris flows?
JK: Absolutely. Fires are getting more frequent. They're getting bigger. The fires are coming closer to where people live. And rainfall is becoming more intense. So even though we're in drought conditions, we're seeing these intense rainstorms.
We expect it to continue. The data shows that in the last 10 years, there's been a real increase in these types of events.
NO: Is there anything that homeowners or property owners can do to mitigate these risks?
JK: The most important thing you can do is not be around when it rains, or maybe more importantly, listen to the National Weather Service.
Every time we do an assessment, we send our numbers to the local weather forecasting office, and we give them this guidance, like, ‘When it rains this hard, things might start to kick off. You should be concerned.’ So we work directly with them, and just like they issue a flash flood watch, they'll issue debris flow warnings, which they did for the Elk burn area for that July 15 storm.
You can't stop them, you can't slow them down. There are barrier structures and diversion channels, but know that a flow could overtake something a homeowner does. If you divert the flow and it goes into your neighbor's yard and whacks their house, you may be responsible.
Our advice is always: You know where water flows downhill. Don't be around that if you're below a burn area.
Your local authorities are well tuned to what's happening. The [Wyoming] Department of Transportation, the emergency management offices, they're really the experts because they know where things happen. If they close roads or if they tell you to evacuate, it's really important to listen.
