Wildfires and Landslides: It’s Not Always a Direct Link

In the wake of a wildfire, there’s often an assumption that burned landscapes will be more susceptible to landslides. but new research from the University of Oregon suggests it’s not always that simple.

The Columbia River Gorge Study

An analysis of the Columbia River Gorge, which runs along the border between Oregon and Washington, shows that steep, rocky watersheds in that area have been prone to debris flows and rockfall for thousands of years. These events didn’t measurably increase after the Eagle Creek Fire,which scorched 47,000 acres of the gorge over the course of three months in 2017.

UO geologist Josh Roering and members of his lab published their findings on August 8 in Science Advances, highlighting the importance of context and geological history in landslide risk assessments. The study also could help inform safety and hazards awareness projects in the gorge, in both burned and nonburned areas.

Why the Concern After Wildfires?

After the Eagle Creek Fire,Oregon land managers were concerned about landslides,especially in the vicinity of the Interstate 84 transportation corridor that runs through the gorge.Their fears were, in large part, informed by what has transpired in places like Southern California, where post-fire slides have caused devastating casualties and millions of dollars in damage.

This phenomenon can happen because wildfire destroys vegetation and groundcover, making slopes more prone to debris movement, erosion, and rockfall. These events can be more easily triggered by rain and storm events.

The Role of Geological History

“When Eagle Creek burned up such a massive area of the Columbia River Gorge, we expected to see a important increase in landslide activity,” Roering said. “But what we found was that these steep,rocky areas have always been prone to these types of events. The fire didn’t seem to change that fundamental pattern.”

The research team used a combination of ancient aerial imagery, field observations, and computer modeling to reconstruct the landslide history of the region. They discovered that debris flows and rockfalls were common in the gorge long before the Eagle Creek Fire.

Key Takeaways

• Landslide risk isn’t solely resolute by wildfires.
• Geological history and terrain play a crucial role in landslide susceptibility.
• Post-fire landslide assessments should consider the specific context of each landscape.
• Areas with a long history of debris flows and rockfalls may not experience a significant increase in these events after a fire.

Implications for Land Management

This research suggests that a one-size-fits-all approach to post-fire landslide risk assessment isn’t effective. Land managers need to consider the unique geological characteristics of each area when developing mitigation strategies.

“we need to move beyond simply looking at burned areas and start thinking about the underlying geology and the long-term history of slope instability,” Roering said. “That will give us a much more accurate picture of where the real hazards are.”