The charred landscapes that wildfires leave behind are susceptible to erosion, flooding and debris flows, like the mudslides that blocked roads in Basalt the summer following the Lake Christine Fire.
Liz Schnackenberg, a hydrologist with the US Forest Service who analyzed the Lake Christine burn area, is in the Roaring Fork River valley this week for the Naturalist Nights speaker series, discussing how wildfires change watershed conditions.
She has looked at maps of watersheds from across the country and world as she helps local communities plan for hazards after wildfires, but maps of the south side of Basalt Mountain threw her for a loop.
“When I first looked at the topographic map for that portion of the fire, I was like, ‘I’m a hydrologist and I cannot figure out which way the water’s flowing. I can’t tell where it’s going, I can’t tell where the streams are, and it’s like, this is weird, I’ve never had this problem before,’” Schnackenberg said.
Once she flew above the burn area in a helicopter, she could see why those topo maps were so befuddling. The terrain on Basalt Mountain is all broken up, with steep slopes draining into flats where it isn’t totally clear where water might flow out.
Now, lessons from Lake Christine are a staple in presentations that she gives about how fires can affect watersheds.
“It’s such a great contrast of what I’ll call more traditional fire effects or more traditional landscapes versus a varied landscape,” she said, explaining that local terrain has a significant influence on the types of hazards that a wildfire leaves behind.
When wildfires burn through vegetation and scorch soils, it leads to increased risk of erosion, flooding and debris flows. The soil holds clues about how communities need to prepare for those post-fire hazards. Badly burned soil doesn’t absorb water; it acts more like pavement, repelling rain so it runs downhill.
But on Basalt Mountain, the unusual topography added a new layer to the hydrology. Flat areas help to slow the momentum of flooding or debris flows.
“Because it’s not a continuous flowpath, it’s just not going to keep building on itself,” Schnackenberg said.
While that made it harder to predict exactly where future flooding might occur, this terrain likely helped protect the town of Basalt during some of last summer’s intense rain storms.
As the vegetation regenerates after the Lake Christine Fire, the potential for extreme flooding in Basalt recedes. Regrowth is key to preventing flooding and debris flows.
“That can be your grasses and forbs, it doesn’t have to be trees,” Schnackenberg said.
Those ground-covering plants help stabilize the soil and encourage the earth to start soaking up water again, which, in turn, reduces erosion.
Grasses started growing back in the burn area within weeks after the flames from Lake Christine died down, and in three to five years, Schnackenberg predicts that much of the burn area will be green again.
For now, she said, it’s important to minimize any more disturbances that might give noxious weeds a chance to outmuscle the native plants. The best thing people can do now is “just give those plants a chance to grow.”
These ecosystems evolved with and adapted to wildfire.
“They’re generally pretty good at bringing themselves back unless there were really substantial effects to the soil,” Schnackenberg said.
Flood, Debris-Flow Risks Remain
Schnackenberg and her colleagues who assessed the severity of the soil damage on Basalt Mountain rated most of the area closest to town as moderately burned, meaning up to 80 percent of the ground cover was consumed by the fire in that area. Even as vegetation is returning and the terrain catches some of the fast-moving water, there’s still a chance for more flooding.
“I certainly would expect some to occur,” Schnackenberg said. “It's not like just because it's a broken landscape, nothing will happen. It's just a matter of extremes.”
Schnackenberg discussed how wildfire can change watershed dynamics Wednesday in Carbondale as part of the Naturalist Nights speaker series; she speaks again tonight/Thursday at 6 p.m. at Hallam Lake in Aspen.
Aspen Journalism collaborates with Aspen Public Radio and The Aspen Times on coverage of water and the environment. More at aspenjournalism.org.