Landslides Striking in Unexpected Places
Northern British Columbia was subject to a rash of large landslides in the past decade, and they happened in places that terrain specialists normally consider stable. On average 2.3 slides moving over half a million m³ of material or spreading over 1 km occurred in the region each year, up from 1.3 annually during previous decades.
At least some of the surge in slides is caused by warmer temperatures melting the permafrost scattered in high mountains and in northeast BC’s muskeg. Ice that has held hillsides together for millennia is now degrading. Once gone, the slopes crumble, as occurred in the Harold Price watershed near Smithers in 2002, triggering a 4 km-long landslide that began on an alpine ridge. On muskeg lowlands near Fort Nelson, the presence of permafrost, let alone its melting, is not obvious on air photos.
Other hidden features have also caused the ground to suddenly give way. Many massive slides stem from ancient topography that is no longer visible. In northeast BC slides recently formed in buried valleys and lake sediments that glaciers during the last ice age filled and obscured with till. Now a gentle landscape of boreal forest conceals the underground topography.
Near the coast, sediments laid down in fiords by ancient marine glaciers have since been uplifted and now sit buried on valley hillsides. These will unravel a sizeable forest in response to a small, innocuous initial landslide. Fragile terrain created by the obscured marine sediments commonly hides near the Nass, Terrace and Kitimat.
Other surprises have frequently arisen from small rockslides that land on shallow, benign slopes then trigger a catastrophic landslide below. On stable slopes of only three degrees huge earth flows have been generated from the energy of falling rocks. One in the Muskwa area of northeast BC travelled over 3 km and covered 180 ha. Saturated earth helps move material over long distances. In the last five years rockslides have triggered earth flows, debris flows and rock avalanches racing downslope anywhere from 1.5 to over 4 km.
Marten Geertsema and James W. Schwab. 2006. Challenges with Terrain Stability Mapping in Northern British Columbia. Streamline Watershed Management Bulletin. 10(1): 18-26.