Modelling fire-induced perturbations in sediment flux based on stream widening and accelerated bank migration

Abstract

Watersheds impacted by wildfires undergo alterations in water, sediment, and biogeochemical fluxes owing to, e.g., soil destabilization and loss in evapotranspiration potential. Enhanced sediment availability and surface runoff are known to make burnt slopes susceptible to localised slope failure, whereas larger watershed-scale (>1,000 km2) changes in sediment flux are largely unconstrained. This shortcoming results from the scarcity of direct measurements from most large, remote watersheds, which makes modelling efforts particularly relevant. Here, we develop a two-fold model to estimate sediment flux from hillslopes to a river based on characteristic rates of catchment erosion and relationships with riverbank migration. We apply this model to the watershed of the Bonaparte River (British Columbia, Canada), which was heavily impacted by the Elephant Hill megafire in the summer of 2017. Photogrammetric data collected from three meandering reaches reveal sharp post-fire channel widening and accelerated migration (up to 130% and 230%, respectively) in the fire’s aftermath. Field observations link these trends to enhanced post-fire suspended- and wash-load fluxes to floodplain reaches, whereas input of bedload sediments was likely restricted to bedrock reaches. Our model predicts a power-law relationship between fire-induced sediment-supply enhancement from hillslopes and riverbank mobility, consistent with the hypothesis that sediment flux affects meander migration. Future integrations of these results with sub-catchment-scale mapping of burn severity and direct data on sediment capacity may help better inform alterations in sediment and biogeochemical fluxes in the aftermath of wildfires elsewhere.