Morphodynamic adjustment in a watershed affected by megafires: Deadman River, British Columbia, Canada

Abstract

Megafires have profound impacts on the movement of water and sediment along watersheds because of changes in the mechanical and hydrologic properties of soil and vegetation. Literature has often focussed on local post-wildfire mass-wasting events, although less attention has been paid to watershed-scale river responses. We present a study from the Deadman River of British Columbia (Canada), which was recently affected by wildfires, including the severe Sparks Lake Fire of 2021. Timelapse photogrammetry and multispectral remote sensing on both pre- and post-fire conditions show how fire-related changes in hydrology and vegetation influenced channel morphodynamics. The photogrammetric analysis indicates a watershed-wide increase in channel width and meander migration rate after the Sparks Lake Fire, with a sharp increase of up to 100% and 540%, respectively. Once normalised by the timespan over which migration rates are averaged, results indicate an up to ~20-fold increase in channel mobility post-fire. The remote-sensing analysis reveals that regions that burned more severely drained into reaches that migrated laterally at faster rates. These results provide a quantification of the degree to which megafires alter the morphodynamics of meandering rivers, with anticipated ramifications for natural-hazard mitigation and ecosystem recovery efforts in affected areas.