Any process which results in the exposure of icy sediment to thaw may induce a landslide. Thawing not only reduces cohesion but may also reduce frictional strength of the sediment especially if ice in excess of the sediment pore volume is present. The water produced by thaw must escape but until it does so, it carries the weight of the slope sediments, decreasing the friction between soil particles. Landslides caused by elevated pore water pressures due to thawing ground ice are common in the Mackenzie Valley. Also, undercutting of river banks can result in enough loading to overcome the cohesion of frozen sediments. Approximately 2,000 slides have been documented in the Mackenzie Valley, with a further 1,000 retrogressive thaw-flows identified in the Mackenzie Delta-Tuktoyaktuk Peninsula area.

Although many landslides in the Mackenzie region are related to earthquakes or river erosion, many are also related to climatic or thermal factors. Ground ice is associated with many, perhaps most, landslides in glacial sediments. Thermally induced instability in permafrost slopes can result from changes in proximity of warm waters, exposure of ground ice by either frost cracking or shoreline erosion, or from the destruction of the insulating organic cover by fire or other causes. Other landslides are directly related to extreme climate events such as abnormally high precipitation or high summer temperatures. Because so much of the Mackenzie Valley contains permafrost and icy sediments are common, any warming, whether annual or longer term, can result in the deepening of the active layer, the melting of interstitial ice, and inducement of a landslide. Heavy rain may directly saturate the ground resulting in mobilization of the slope, or increase the pore water pressure to the point of instability, or rain-swollen rivers may erode the base of their banks, inducing landslides.

In the Mackenzie area the frequency of forest fires is significantly increased during droughts. If ground conditions are very dry, the entire organic mat can be burnt, exposing icy sediments, leading to an active layer detachment failure, and possibly mobilizing the slope in a rapid debris flow. Even under present climatic conditions a definite link between fire and landslides can be made. Following the large forest fires of 1994 and 1995 in the vicinity of Fort Norman, numerous flows developed along the banks of the Mackenzie River and its tributaries. Thus, if fire frequency increases in the future, the frequency of skin flows can be expected to increase.

[Click on an image thumbnail to view a larger image, notice]

Active layer detachment slides which occurred along the Mackenzie River bank between Norman Wells and Tulita in an area burnt by forest fire in June 1995. Landslides began in late summer of 1995. Photo taken Sept. 1996.	Block landslide in permafrost along Mackenzie River, near Old Fort Point. Landslide occurred in the winter of 1997 and flowed onto the frozen river. Photo taken March 1997.
Block landslide in permafrost along Mackenzie River, near Old Fort Point. Landslide occurred in the winter of 1997 and flowed onto the frozen river. Photo taken March 1997.	Same block landslide in permafrost along Mackenzie River, near Old Fort Point. Photo taken in June 1997, shows broader view of river bank. Note that large frozen blocks visible in winter are no longer distinct and have begun to thaw and flow.
Same block landslide in permafrost along Mackenzie River, near Old Fort Point. Photo taken in June 1997, shows broader view of river bank. Note that large frozen blocks visible in winter are no longer distinct and have begun to thaw and flow.

Text extracted from Aylsworth & Duk-Rodkin (1997) & Dyke et al. (1997).