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 km 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 (10- to 10-year) erosion from debris fans and volcanic edifice reconstruction. Fan-derived 10-year erosion rates (1 to 9 mm year) exceed 10-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 10-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.