NEW INSIGHTS INTO FORM AND FUNCTION OF VERY SLOW-MOVING LANDSLIDES FROM BATHYMETRIC SURVEYS IN THE THOMPSON RIVER VALLEY, BRITISH COLUMBIA

Compound translational landslides along Thompson River, British Columbia, between 50.75° and 50.60° N have been periodically active since at least the 1860s, posing risks to vital rail lines, communities, sensitive ecosystems, and freshwater resources. Several of the landslides experienced large, rapid failure events in the twentieth and late nineteenth centuries during the late summer and fall. Our ongoing investigations provide new insight into the geological and fluvial controls on landslide activity in this corridor. The landslides move on sub-horizontal rupture surfaces generally below river level in impermeable clayey glaciolacustrine containing silt beds and sandy lenses and overlying bedrock. Their creep forms tension cracks, active wedges, and lateral shear zones that increase the hydraulic connectivity and hydraulic heterogeneity of the slide masses. Displacement monitoring since2008 has revealed that during the summer when river and groundwater levels are highest, landslide creep is minimal. When river and groundwater levels are low during the winter, landslides tend to destabilize and creep rates increase. This seasonal acceleration is probably in part a response to the loss of toe support and changes in pore water pressures within lower valley slopes. Large failure events, in contrast, have long been speculated to be irrigation triggered. We have mapped deep (> 2 m to <50 m) scour pools at the base of all the Thompson River landslides along the study reach. This pool and riffle channel morphology directly influences the location and activity of landslide since hydraulic conductivity between the river, landslides and valley-slope groundwater system is likely to be greatest near pools, where fractured bedrock and hydraulically heterogeneous glaciolacustrine units are continually exposed by fluvial erosion. We have also identified scour pools along channel stretches downslope of irrigated benchlands. These terraces are potentially unstable and should be monitored together with active landslides for surface subsidence, tension cracks and scarps to improve characterization and management of geohazards in the Thompson River valley, and ultimately mitigate risks to public safety, the environment, and Canada’s economy.