The landslide response of alpine basins to post-Little Ice Age glacial thinning and retreat in southwestern British Columbia
Introduction
The extent of alpine glaciers in SW British Columbia has fluctuated significantly since the end of the Pleistocene Epoch. Within the Holocene, at least three major Neoglacial advances have scoured valley bottoms and cirque basins, most recently during the Little Ice Age (Ryder and Thomson, 1986). Twentieth century climatic warming has since caused substantial glacial retreat in most mountain areas (Fitzharris et al., 1996). This paper investigates one of the potential impacts of this change, namely an increase in the pace of landslide activity in alpine basins following glacial retreat.
Several authors have cited recent glacial retreat as a causative factor of slope instability in alpine basins, related primarily to debuttressing of bedrock slopes and deposition of steep glacial drift blankets in areas prone to instability Bovis, 1990, Evans and Clague, 1994, Abele, 1997, Berrisford and Matthews, 1997, Haeberli et al., 1997, Ryder, 1998. Although the areas affected by such instability often lie far beyond human settlement and infrastructure, large-scale slope movements such as highly mobile rock avalanches, debris flows, and outburst floods may cause extensive destruction far downstream of their initiation zones. For example, the Meager Creek area of SW British Columbia contains several areas subject to such hazards, including a popular hotsprings recreational site. In October 1931, a debris flow estimated at 5×106 m3, initiated in recently deglaciated terrain in upper Devastation Creek, a tributary to Meager Creek (Carter, 1931). The debris flow traveled 15 km along Meager Creek to the Lillooet River confluence and would have overwhelmed the present hotsprings facilities. In 1975, another large (1.4×10 m7) debris flow, triggered by the collapse of a glacially debuttressed slope, descended Devastation Creek, (Evans and Clague, 1988), claiming four lives on Meager Creek floodplain. The access road to Meager Creek hotsprings is also frequently blocked by debris flows originating in recently deglaciated terrain in Capricorn Creek, including a 1998 event (Bovis and Jakob, 2000) that necessitated the helicopter rescue of several people trapped at the hotsprings site.
Most studies to date have focused on specific landslide events linked to recent glacial retreat (e.g. Bovis, 1990, Blown and Church, 1985), but so far, there has been no regional scale investigation of the effects of post-LIA glacial retreat. We analyze the effects of post-LIA glacial retreat on landslides in 19 alpine basins near Pemberton, British Columbia (Fig. 1). This work serves to prioritize areas that may become unstable in the future as glaciers retreat still further. Such events have the potential to threaten settlements and sensitive infrastructure which have not yet been affected by landslides. Of particular importance in western Canada, as in many other mountainous glacierized regions of the world, are transportation, telecommunications, and pipeline corridors.
The objectives of this study are to identify the terrain characteristics associated with landslides in bedrock and surficial material, and to determine the glacial influence on slide initiation by identifying how Neoglacial scour and retreat have modified hillslope form and slope stability. Results are based on a GIS analysis and statistical interpretation of field data via a process-oriented view of slope instability. We also present a decision flowchart to assist in the identification of landslide hazards associated with recent glacial retreat.
Section snippets
Previous work
Several workers have noted the association between Neoglacial retreat and slope movement processes, such as gravitational rock slope deformation, rock avalanches, debris flows, and debris slides. In the Coast Mountains of British Columbia, there are several documented instances of sagging (sackung) features in areas where recent ice retreat has removed buttress support to glacially undercut slopes Bovis, 1982, Bovis, 1990, Evans and Clague, 1994, Bovis and Evans, 1996, Bovis and Stewart, 1998.
Study area
The 19 alpine basins examined in this study occur in a 500-km2 block of rugged mountain terrain within the Lillooet River watershed, NW of Pemberton, BC (Fig. 1). All basins are located in the transition between wet maritime conditions on the west flank of the Coast Mountains and the drier, submaritime climate to the east. Precipitation ranges from 750 to 1000 mm annually. Basins were chosen at approximately similar elevations and in the same climatic region to minimize climatic bias. All
Methods
In this study, we identify the terrain characteristics responsible for landslide initiation in recently deglaciated areas and determine the degree of glacial control. The sources of information and major steps in this work include field mapping, GIS analysis, statistical associations between landslides and terrain attributes, and comparisons between Neoglaciated and non-Neoglaciated terrain within each basin.
Failures in surficial material
In the classification tree analysis, “Surficial Material Type” and “Slope Gradient” provide the best discrimination between landslide and non-landslide locations (Fig. 4). Terrain most commonly associated with surficial failures includes rock slopes steeper than 30° mantled by till or colluvium, and thick glacial drift deposits, such as lateral moraines. Surficial mantles overlying rock are termed rock-controlled slopes where it is apparent that the underlying bedrock controls the slope form.
Conclusion
Post-Little Ice Age glacial retreat is one of many factors influencing landslide activity in the southern Coast Mountains of British Columbia. Other factors include earlier Pleistocene glacial erosional episodes, rock structural and lithological controls, and undercutting of slopes by fluvial erosion. The evidence presented here suggests that post-LIA glacial retreat has increased the spatial frequency of surficial failures within the Neoglacial limit, has increased rock fall occurrence along
Acknowledgements
This research has been funded by an operating grant to M.J. Bovis from the Natural Sciences and Engineering Research Council of Canada. We wish to thank Carolyn Saunders for assistance in the field. We also gratefully acknowledge the helpful comments from reviewers Jon Harbor and Harry Foster.
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