The influence of wildfire on debris flows in a landscape of persistent disequilibrium: Columbia River Gorge, OR, USA

Extensive debris flow fans emanate from steep catchments in the Columbia River Gorge (CRG), Oregon, a landscape dramatically reshaped by Late Pleistocene megafloods. In 2017, the Eagle Creek Fire burned 200 km² of the CRG, prompting concerns of heightened debris flow activity, yet its impact on hazard potential remains unclear. To assess the fire's effect on 10 CRG catchments, we quantify pre- and postfire debris flow erosion using airborne lidar, imagery, and field observations, as well as long-term (10⁴- to 10⁶-year) erosion from debris fans and volcanic edifice reconstruction. Fan-derived 10⁴-year erosion rates (1 to 9 mm year^-1) exceed 10⁶-year rates by 10 to 50 times, suggesting sustained, rapid unraveling of these catchments following the megafloods. Pre- and postfire debris flow erosion rates are consistent with 10⁴-year rates, such that fire-derived erosion accounts for a relatively small fraction of post-megaflood denudation (<10%), implying persistent landscape disequilibrium that manifests as ongoing high hazard potential in the CRG, regardless of wildfire conditions.