Landslides

Slopes affected by landslides are common in southern Saskatchewan. Landsliding, the downslope movement of soil and rock under the force of gravity, is an important geological process and natural hazard in the valleys of the interior plains of western Canada. The region is characterized by poorly consolidated bedrock and Quaternary deposits which are especially susceptible to landsliding. The original low strength of the sedimentary bedrock, particularly Cretaceous shales, was further reduced as the rocks expanded following unloading. This occurred first as overlying sediments were removed by erosion in the late Tertiary, and then with the wasting of the Pleistocene ice sheets. While this earth history predisposed the bedrock to landsliding, the failure of a slope nearly always requires a triggering event. The undercutting of a valley side by stream erosion is a common trigger, although most landslides are caused by heavy rain or snowmelt. Soil and weathered rock, especially those containing clay (shale, for example) have much lower shear strength when the pore spaces between minerals are saturated with water. Most of the landslides in Saskatchewan occur in Cretaceous shales such as the Bearpaw Formation. Slope stability in the shale bedrock is understood largely from geotechnical studies, notably those associated with the constructions of the Gardiner and Qu'Appelle dams, which impound Lake Diefenbaker as part of the South Saskatchewan River Project.

Landsliding is the dominant process of valley widening. Individual landslides can extend for several kilometres along a valley side and cover tens of square kilometres. Geotechnical studies in the Frenchman River Valley north of Climax revealed that the valley is about 80 m less deep and almost three times wider than the original glacial meltwater channel; the valley fill is composed largely of landslide deposits. Nearly all the landslides in Saskatchewan are inactive and appear very old, suggesting that there must have been widespread slope failure associated with the initial downcutting of the valleys following retreat of the continental ice sheet. Recent landslides usually have reactivated old slides or are part of the process of scarp retreat. In 1956 a landslide along Echo Creek Valley near the town of Fort Qu'Appelle severed Highway 35 in a “rapid flow” movement that is rare in Saskatchewan. Many apparently inactive landslides may be moving at rates too slow to detect without careful monitoring (e.g., the Lebret slide in the Qu'Appelle Valley); they are easily reactivated when disturbed, typically by excavation or the shifting of streams. Landslides expose poorly consolidated materials to runoff erosion for years or even decades. Sediment-laden runoff from landslides affects the geometry and Water Quality of nearby streams, producing downstream impacts that often represent more significant hazards than the direct effects of landslides. Where landslides extend onto the valley floor, streams are impounded or diverted.

David Sauchyn

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Further Reading

Eckel, B.F., E.K. Sauer, and E.A. Christiansen. 1987. “The Petrofka Landslide, Saskatchewan,” Canadian Geotechnical Journal 24 (1): 81-99; Sauer, E.K. and E.A. Christiansen. 1987. “The Denholm Landslide, Saskatchewan, Canada: An Update,” Canadian Geotechnical Journal 24 (1): 163-68.