The role of wildfires and forest harvesting on geohazards and channel instability during the November 2021 atmospheric river in southwestern British Columbia, Canada

Abstract

Sediment mobilized to rivers during extreme flood events can influence channel stability and cause significant morphological changes. A prolonged and intense atmospheric river (AR) struck southwestern British Columbia, Canada in November 2021, leading to extreme flooding and landsliding over approximately 70 000 km² of mountainous areas. Entire communities within the region were evacuated, and the transportation infrastructure connecting them was severely damaged. The locations of 1300+ geohazards (e.g., debris flows, debris flood, debris slides, shallow landslides and bank erosion) were mapped from helicopter, ground observations, orthoimagery, site photos and social media posts alongside rivers and large gravel-bed streams that experienced lateral instability. Morphological changes in two of these gravel-bed rivers were examined in more detail by comparing pre-event and post-event lidar data using three-dimensional point-based normal differencing. We found that geohazards occurred more frequently in burned areas and along forest harvesting resource roads, providing point sources of sediment that entered mainstem rivers. The geohazard mapping and lidar change detection revealed that bank erosion and lateral instability often occurred downstream of these mapped sediment sources. As the frequency of wildfires and extreme meteorological events is predicted to increase with continued climate change, future risk assessments in communities should consider sediment sources that can be mobilized by these events and the resulting downstream morphological impacts.