The effects of a wildfire are not over once the flames are extinguished. Especially when a fire rages on steep and varied terrain, as is common in Colorado’s mountains and foothills, the risk of flash floods and mudslides in the area can be elevated for years to come.

That’s the case this week in Larimer and Boulder counties, where flash flood warnings were issued Wednesday for the Alexander Mountain and Stone Canyon fire areas, respectively. The danger remains, with rain forecast through the end of the week.

Deadly flash floods hit the community of Glen Haven near the Cameron Peak fire area in 2022, nearly two years after the largest wildfire in state history burned more than 200,000 acres of land.

The phenomenon is due to the changes in the landscape caused by the fire. We asked Peter Nelson, associate professor in the Department of Civil and Environmental Engineering at Colorado State University, to help explain these changes.

“When a landscape burns, the landscape’s response to rain changes dramatically,” Nelson said. “You can think of the soil as a sponge. Under normal, unburned circumstances, the sponge – the soil – can absorb much of the rain that falls through infiltration. But after a fire, the landscape’s infiltration capacity is severely limited.”

Organic materials on the surface, such as grass, brush and leaves, are burned in the fire. During an intense fire, compounds form in the top layer of soil that cause the soil to repel water, according to the National Weather Service.

The scientific term for this is “hydrophobia.” Nelson compared it to water rolling off the ground, much like water rolling off the back of a duck.

When it rains, people living downstream or below the burn areas need to be most careful, Nelson says, because those areas are likely to experience flooding from water runoff from the burn scars.

National Weather Service

What if the landscape burns down twice, in separate fires?

Parts of the northern boundary of the Alexander Mountain burn area actually overlap with parts of the southern edge of the Cameron Peak burn scar from four years ago.

Jason Gruenauer of Denver7 asked the question: Is it possible to double the impact on the landscape?

Nelson said we may learn the answer to that question after the Alexander Mountain Fire, as such an overlap is rare. But he explained that the water resilience of the landscape depends on the intensity of the flames, and in this case, a worsening of flood risk in the areas that burned twice is unlikely.

“The hotter the fire was – the more severe the fire – the worse these hydrologic responses tend to be,” he said. “So if the area that burned in Cameron Peak in 2020 had less fuel in this new fire, it might not have burned quite as severely this time.”

What can be done to reduce the risk of flooding?

Nelson is part of a group that has been studying the impact of mulch drops after wildfires. Helicopters drop mulch over a fire area in a large-scale operation that helps reforest the forest and reduce erosion and mudslides. This method was used after the 2020 fires.

But does it work? Nelson said his team is still collecting data on the impacts of mulch deposits across Cameron Peak and East Troublesome Scars. He discussed the early results of the research.

“It depends on where it’s applied, at what concentration and what type of aerial cover,” he explained. “I think our findings from this research so far are that the sooner you can do it, the more likely it is to be successful. The more mulch you can apply to a landslide of concern, the more likely it is to be effective.”

When we spoke with Nelson on Thursday, dropping mulch before rainy days was not yet an option in the Alexander Mountain fire region because the fire was not yet fully contained, but he said it was likely an option that would be considered during recovery from the fire after full containment.