Hydrogeomorphic response of steep streams following severe wildfire in the Western cascades, Oregon

Abstract

Severe wildfire may alter steep mountain streams by increasing peak discharges, elevating sediment and wood inputs 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. We evaluate the hydrologic and geomorphic response of ~100-m-long steep stream reaches to the large-scale and severe 2020 fires in the Western Cascade Range, Oregon. In the two runoff seasons after the fires, peak flows in burned reaches were below the 2-year recurrence interval flood, a level sufficient to mobilize the median grain size of bed material, but not large enough to mobilize coarser material and reorganize channel morphology. Sediment inputs to study streams consisted of two road-fill failure landslides, slumps, sheetwash, and minor bank erosion; precipitation thresholds to trigger debris flows were not exceeded in our sites. There was a 50% increase in the number of large wood pieces in burned reaches after the fires. Changes in fluxes of water, sediment, and wood 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, and an unburned reference reach exhibited little morphologic change. Post-fire recruitment of large wood will likely have long-term implications for channel morphology and habitat heterogeneity. Below-average precipitation during the study period, 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 increasing the propensity for both drought and extreme rain events and by altering vegetation recovery patterns.