Post-wildfire permafrost landslides and cascading hazards, Dempster Highway,Yukon

Yukon Territory is experiencing impacts of climate change, marked by elevated annual air temperatures, changes in precipitation patterns and increased wildfire activity. These shifts can lead to permafrost degradation, impacting highways and community infrastructure. This study characterizes the timing and morphology of post-wildfire permafrost landslides and documents a cascading hazard. It identifies relationships between permafrost characteristics, geology, weather conditions and ground disturbance. This work contributes to the growing body of research on how climate change is impacting communities and infrastructure in permafrost regions.

In 2017, a wildfire burned across a slope, underlain by permafrost, parallel to the Dempster Highway in northeastern Yukon. Within days, multiple active layer detachments (ALDs) occured caused by degradation of the insulating organic surface layer resulting in rapid permafrost thaw. Over forty ALDs occurred on the slope over the summers of 2017 and 2018, likely influenced by rainfall events and periods of above average air temperatures. Initiation angles for ALDs varied according to surficial geology. Areas with shale-rich colluvium had initiation angles as low as 10° while in sandstone dominated colluvium, initiation angles were greater than 25°. By 2019, portions of the slope appeared to stabilize as no new ALDs occurred; however, six retrogressive thaw flows (RTFs) initiated in ALD landslide scars. RTFs only occurred on topographic benches where ice-rich stratigraphy had been exposed by complete removal of the insulating surface organic layer by the ALD. The headwalls of the active RTFs consist of metre-scale ice wedges, as well as loess and organic-rich colluvium units. OSL ages indicate sediments accumulated over the last ~100,000 years. The surficial units were sampled and measured for volumetric and gravimetric ice-content. The ice content generally increased with depth.

RTFs have deposited significant amounts of sediment on the floodplain at the base of the slope near the highway, and four of the RTFs were still active during site investigations in the summer of 2023. The increased sedimentation in the valley bottom has led to stream blockages and flooding, degrading permafrost beyond the perimeter of the original burn. This research indicates complex cascading hazards can occur in permafrost areas due to anthropogenic global warming. At this site we document a forest fire, that triggers abundant ALDs, some of which then evolve into RTFs, which generate abundant sediments, blocking drainages and causing flooding which will likely trigger more permafrost degradation. This research indicates that wildfire on permafrost slopes can initiate a cascading hazard that can be further influenced by local precipitation and warm summer temperatures.