Hydrogeomorphic Response of Steep Streams following Severe Wildfire in the Western Cascades, Oregon

Severe wildfire may alter the morphologic resilience of steep mountain streams by increasing peak discharges, elevating inputs of sediment and wood into channels, and increasing susceptibility to landslides and debris flows. In the Pacific Northwest, where mean annual precipitation is high and mean fire return intervals range from decades to centuries, understanding of steep stream response to fire is limited. In 2020, the Western Cascade Range, Oregon, experienced wildfire of historic magnitude and severity. The objective of this study was to evaluate the hydrologic and geomorphic response of steep streams to the 2020 fires. I assessed streamflow, instream wood, and changes in channel topography and bed-material size in steep stream reaches that burned in the fires and in an unburned reference reach. In the 1.5 years after the fires, peak flows in burned sites were below the two-year recurrence interval flood. A ~5-year flood in the unburned reference reach highlights the influence of other processes besides wildfire in driving streamflow in mountain watersheds. Sediment inputs to streams consisted of two large landslides initiated from road fill failure, slumps, sheetwash, and minor bank erosion. There was a 50% increase in the number of large wood pieces in burned sites after the fires. Changes in fluxes of water, sediment, and wood in burned streams induced shifts in the balance of sediment supply to transport capacity, initiating a sequence of sediment aggradation and bed-material fining followed by erosion and bed-material coarsening. Gross channel form showed resilience to change. The unburned reference reach exhibited little morphologic changes. Post-fire recruitment of large wood will have long-term implications for channel morphology and habitat heterogeneity. Anthropogenic climate change likely contributed to below-average precipitation during the study period which, combined with an absence of extreme precipitation events, was an important control on channel responses. Climate change may have a complex effect on stream response to wildfire by reducing total precipitation while simultaneously increasing susceptibility to flooding and debris flows via prolonged vegetation recovery and projected increases in extreme precipitation