Wildfires Leave Landscapes Prone to Erosion
The first substantial rainstorm after a severe forest fire can trigger large-scale erosion, even on shallow slopes.
This puts anyone located downstream on alluvial fans, where landslides deposit mud and debris, at great risk.
After the wildfires of 2003, large areas of land eroded near Falkland, Kelowna, Cranbrook and Kootenay Lake in southern British Columbia.
Only extremely hot burns, that consume most of the soil’s organic material and leave exposed mineral ground, can create water-repelling soils that are prone to erosion. When precipitation hits the ground, instead of percolating in, water runs off the surface in unprecedented volumes, carrying soil with it.
Conditions become exacerbated when a crown fire also eliminates the vegetation canopy that intercepts rainfall. Even where less than two hectares are in this state, dry weather followed by an intense rainstorm can generate massive erosion and flooding.
Most slides occur shortly after a wildfire, followed by more erosion during the following summer. It takes 2 to 6 years for the soils’ hydrophobic coatings to degrade, allowing water to infiltrate normally. Regrowth of plants also helps reduce erosion, although vegetation can establish slowly on severely burned soils.
Mike P. Curran, Bill Chapman, Graeme D. Hope and David Scott. 2006. Large-scale Erosion and Flooding after Wildfires: Understanding the Soil Conditions. Technical Report 030. BC Ministry of Forests and Range, Forest Science Program, Southern Interior Forest Region. Kamloops, BC. Research Report