Landslides at the south end of Kluane Lake, Yukon Territory

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Landslides are unusually varied and abundant in the Kluane Ranges near the south end of Kluane Lake, Yukon Territory. Selected landslides were investigated to determine the likelihood and probable character of future mass movements in this area, and to gain some understanding of similar but unstudied features elsewhere in the St. Elias Mountains.Landslides in the study area include slumps and related complex landslides, rockfalls, rockslides, rockfall avalanches, and debris flows. The youngest and most spectacular of the large catastrophic landslides is the Sheep Mountain rockfall avalanche (5–10 × 106 m3), which formed as a result of two separate failures between 500 and 1950 radiocarbon years ago. Much of the low mountain slope southwest of this landslide to near the mouth of Slims River is covered by thick blocky rubble deposited during one or more older catastrophic slope failures. Both the Sheep Mountain landslide and the set of older slope failures to the southwest apparently occurred when Kluane Lake was much smaller than it is today. As the level of the lake rose in response to aggradation accompanying the Neoglacial advance of Kaskawulsh Glacier, distal portions of these landslides were inundated.Debris flows and debris torrents occur sporadically on fans in the study area. These fans are composed of diamicton and gravel beds separated by loess layers and paleosols. Marker horizons, such as the Slims Soil (Hypsithermal) and White River tephra (ca. 1200 years BP), occur in these sediments and provide evidence that the fans have been active throughout the Holocene.Contributing factors to landslides in the eastern Kluane Ranges include high seismicity, the presence of steep slopes in pervasively fractured and faulted rocks, an abundance of talus and glacial sediments available for remobilization as debris flows and debris torrents, and the occurrence of intense rainstorms.Although landslides are ubiquitous in the south Kluane Lake area, most of the large deep-seated bedrock failures are relatively old. Thus the danger posed by future comparable landslides to life and property in the area could be considered to be low. Floods, debris flows, and debris torrents on active alluvial fans and aprons skirting the Kluane Ranges probably are greater potential hazards to economic development of this region.