ABSTRACT: Studies of an active rock glacier, East Side, Slims River Valley, Yukon Territory, Canada

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Abstract

The active rock glacier found on the east side of the Slims River Valley, Kluane National Park, Yukon Territory was studied between 1984 and 1986. The work involved measuring the thermal and moisture characteristics of the surface layers of the rock glacier sampling the H2O associated with the rock glacier (rain, snow, spring water, interstitial ice) for isotopic and chemical analyses, monitoring spring discharge at the terminus of the rock glacier and establishing a survey grid for measuring rate of movement. The results have led to recommendations concerning the construction and maintenance of a proposed access road which must pass the active terminus of the rock glacier. An ice-rich diamicton forms the bulk of the lower part of the rock glacier which is overlain by up to 53 cm of loess. The lower two thirds of the rock glacier surface is covered by a mature spruce forest community. Active layer thicknesses on the rock glacier surface range from 30 cm to 220 cm, depending on site conditions. Active layer thicknesses have decreased by as much as 20 cm over the last 30 years. Permafrost is absent in the floodplain silts and sands bordering the front of the rock glacier except in the push lobe found along the north facing exposures of the terminus and also in seasonal frost mounds west of the rock glacier. The near surface hydrology of the rock glacier consists of throughflow in the surface layers over the essentially impervious permafrost below. This throughflow manifests itself as four springs and as zones of seepage along the front of the rock glacier. Maximum discharge rates for a single spring reached 195 litres/min. No diurnal trends in spring discharge were observed with only major precipitation events showing up as peaks in the discharge hydrographs. Discharge over the course of the summer season generally mimicked overall trends in air temperature. Interstitial ice found in the rock glacier is derived from meteoric sources such as snowmelt and rainfall. Ground temperature and moisture data indicate that thermal gradients provide the requisite force carrying a net downward movement of moisture into the upper layers of the permafrost from the active layer. The process of moisture migration is regarded as the primary mechanism leading to the accumulation of ice within the debris of the rock glacier and this is the first time that the origin of the ice has been established in active, near-slope rock glaciers. Based on a present length of 1700 meters and the assumption that the rock glacier formed 12500 years B.P., the front has advanced at an average rate of 13.6 cm/yr. From May 1984 to June 1986 portions of the front have advanced at rates between 1 and 18.5 cm/yr, averaging 6.7 cm/yr. Movement on the surface of the rock glacier approaches 240 cm/yr where slopes are steeper some 300 meters upslope from the snout. These high surface velocities and low rates of frontal advance indicatecompressive flow is occurring and this is shown by the presence of well developed transverse ridges and furrows. Internal shearing within the body of the rock glacier may also be indicated by this compressive flow. Parks Canada have proposed the construction of an access road into Kluane National Park which must pass the front of the rock glacier. This study recommends that the road pass over the rock glacier terminus rather than in front of it. This choice minimizes the environmental impact as well as offering a simpler and cheaper alternative.

Bibliography of Canadian Geomorphology