Five case studies of recent landslides in southwestern Yukon Territory illustrate the role of permafrost in landslide processes of the region. In the Marshall Creek basin, permafrost degradation after recent forest fires caused numerous debris flows near the valley bottom. Similarly, on Haeckel Hill, firerelated deepening of the active layer has facilitated active layer detachment slides on upper hillside slopes. In the Kluane Range, the interface between frozen and unfrozen ground appears to control the depth of movement for active layer detachment slides and debris flows along Silver Creek. The failure mechanism on Mount Sumanik is controlled by a frozen substrate, which contributes to a reduction in drainage and elevated pore-water pressure. Lastly, thawing of segregated ice has caused a thaw slump of fine-grained sediment in lacustrine terraces along Takhini River.

A reconnaissance inventory of permafrost-related landslides in the Pelly River watershed was conducted in 2006, largely in response to local community concerns regarding the potential impacts of climate change on slope stability and possible effects on water quality. Using aerial photograph analysis, satellite imagery, and visual inspection from a fixed-wing aircraft, over 100 permafrostrelated slides were located near the Pelly and MacMillan rivers and various tributaries. Basic geomorphic characteristics were determined for many of the failures based on analysis of remote sensing data, and reviews of existing literature and surficial geology maps. Most of the landslides identified were small active-layer detachments and retrogressive thaw failures. Several large failures also illustrate important characteristics associated with permafrost-related landslides, including their source-area setting, triggers, high mobility, the longevity of their activity and their ability to impact very large areas. The nature and distribution of the identified failures highlights a number of implications for land-use in central Yukon and emphasizes the need for enhanced methods of permafrost detection and regional mapping in the Territory.

The Yukon is experiencing impacts of climate change, marked by elevated annual air temperatures, alterations in precipitation patterns and increased wildfire activity. These changes can lead to permafrost degradation, impacting highways and community Infrastructure. In July 2017, a wildfire burned a slope in permafrost terrain above the Dempster Highway in the Yukon. In the years following the wildfire, two types of permafrost-related landslides have been observed on the slope. Active layer detachment activity was highest in the first year after the landslide, possibly influenced by warm temperatures and rainfall events. Retrogressive thaw flow slides formed in 2019 in areas of ice-rich permafrost and are still active in 2023. Deposition of sediment and influx of water has resulted in flooding near the highway, further degrading the permafrost in the valley bottom. This study characterizes the landslide timing and morphology following a wildfire on permafrost terrain, and investigates potential triggers and controls.

The following report describes the settings, causes and geological controls of landslides in the Alaska Highway corridor. Although diverse geologic, geomorphic and climatic environments exist in the region, most landslides are related to the presence of shallow bedrock or permafrost, unconsolidated sediment on steep slopes, weak bedrock, groundwater hydrology, river erosion or the degradation of ice-rich permafrost. Where geologic controls provide appropriate settings, intense rainfall, rapid snow melt and seismic events play important roles in triggering failures. Rainstorms that reach thresholds of combined intensity and duration have triggered abundant shallow landslides within the corridor. Debris flows have historically posed the highest risk to lowlying regions and are capable of damaging settlements and transportation routes. The Shakwak Valley has the highest concentration of landslides within the corridor due to the abundance of steep slopes, high relief and widespread discontinuous permafrost. In Wellesley Depression, shallow permafrost and its subsidence has an important influence on slope instabilities. Landslides in the Yukon Plateau primarily relate to the presence of silt- and ice-rich tills on steep valley sides as well as the incision of fine-grained lacustrine terraces in valley bottoms. Debris flows after intense rainfall events are the most common form of landslide in the Kaska Mountains. Finally, in Liard Lowland, failures associated with glacial meltwater and modern stream incision are the most common landslide events. Permafrost plays an important role in landslide processes in the corridor due to its influence on soil moisture, drainage and strength. Slopes composed of icy sediment that have been burned by forest fires are particularly vulnerable to rapid mass movements due to permafrost degradation. The consequences of the dramatic increase in landslide potential after fire should be considered in local fire management plans. The climate¿s local and regional influence on hydrology, fire frequency and permafrost distribution greatly affects landslide processes. Current climate change projections call for warmer temperatures and increased precipitation for the Yukon in the next half century. Among the anticipated effects of global warming in southern Yukon, increased incidents of intense snowmelt and/or precipitation events, river migration, permafrost degradation or forest fires may lead to an increase in landslide frequency and/or magnitude within the settings described in this report. The most significant impact of increased landslide activity may not be a direct impact. Rather, increased sediment input from landslides will likely increase stream channel instability and flooding. This would be particularly acute in the vicinity of alluvial and colluvial fan complexes along Kluane Lake where highway maintenance is already a challenge.

In August 2008, reactivation of the Little Salmon Lake landslide occurred. During this event, hundreds of conical mounds of variable size and composition formed in the deposition zone. The characteristics of these landforms are described and a potential mechanism for their formation is proposed. A preliminary slope stability analysis of the 2007 Mount Steele rock and ice avalanche was also undertaken. The orientation of very high persistence (>20 m long) structural planes (e.g., faults, joints and bedding) within bedrock in the source zone was obtained using an airborne-LiDAR digital elevation model and the software COLTOP-3D. Using these discontinuity orientation measurements, kinematic, surface wedge and simple three-dimensional distinct element slope stability analyses were performed.

Local-scale surficial geology mapping was completed as part of a community hazards mapping program coordinated by the Northern Climate ExChange (Yukon Research Centre, Yukon College). This program assesses potential landscape hazards under changing future conditions by incorporating a variety of data sets, including surficial geology, topography (slope and aspect), permafrost distribution, site-specific permafrost data (e.g. ground penetrating radar, electrical resistivity tomography and borehole data), analyses of past hydrological and climatological trends, and future climate projections. The surficial geology map describes surface landscape features, sediment texture, genetic material, surface expression and geomorphological processes. Detailed descriptions of local surficial geology and hazard analysis methodology are presented in the accompanying report. The accompanying landscape hazard classification map identifies existing and potential geological hazards such as landslides, permafrost stability and flooding; the hazard map is presented in stoplight colours to provide an intuitive tool for community decision makers aiming to incorporate an adaptation planning framework into existing land use management practices.

Results of surficial geology investigations in Wellesley basin and the Nisling Range can be summarized into four main highlights, which have implications for exploration, development and infrastructure in the region: 1) in contrast to previous glacial-limit mapping for the St. Elias Mountains lobe, no evidence for the late Pliocene/early Pleistocene pre-Reid glacial limits was found in the study area; 2) placer potential was identified along the Reid glacial limit where a significant drainage diversion occurred for Grayling Creek; 3) widespread permafrost was encountered in the study area including near-continuous veneers of sheet-wash; and 4) a monitoring program was initiated at a recently active landslide which has potential to develop into a catastrophic failure that could damage the White River bridge on the Alaska Highway.

Local-scale surficial geology mapping was completed as part of a community hazards mapping program coordinated by the Northern Climate ExChange (Yukon Research Centre, Yukon College). This program assesses potential landscape hazards under changing future conditions by incorporating a variety of data sets, including surficial geology, topography (slope and aspect), permafrost distribution, site-specific permafrost data (e.g. ground penetrating radar, electrical resistivity tomography and borehole data), analyses of past hydrological and climatological trends, and future climate projections. The surficial geology map describes surface landscape features, sediment texture, genetic material, surface expression and geomorphological processes. Detailed descriptions of local surficial geology and hazard analysis methodology are presented in the accompanying report. The accompanying landscape hazard classification map identifies existing and potential geological hazards such as landslides, permafrost stability and flooding; the hazard map is presented in stoplight colours to provide an intuitive tool for community decision makers aiming to incorporate an adaptation planning framework into existing land use management practices.

A large block slide affecting an area of approximately 1.0 square km was reported to have occurred on Cement Creek Y.T. (115G/5) between February 15 and March 15, 1983. Landsliding is confined to basaltic andesites in the lower unit of the Wrangell Lavas. Hydrothermal alteration of flow-top breccias and the tilting of strata towards an open face bordering Cement Creek predispose rock to slip along bedding planes. Surveys conducted during June to August 1986 detected displacements of 10-12 cm during an 8 week period. Seismic refraction and electrical resistivity surveys conducted on the slide mass suggest that most ground failure was caused by strong ground motion during an initial episode of rapid displacement and that the rupture surface lies beneath the lower limit of permafrost. Ground-water accumulation on and above the rupture surface followed by landslide initiation during a burst of low magnitude seismicity is suggested as a slide mechanism.