Hydrogeomorphic Response of Steep Streams after Wildfire in the Western Cascades, Oregon
Presentation Type
Oral Presentation
Category
STEM (science, technology, engineering, mathematics)
Abstract/Artist Statement
Severe fire may increase the vulnerability of steep mountain streams (5-10% slopes) to disturbance by increasing peak discharges, elevating inputs of sediment and wood into channels, and increasing the likelihood of landslides and debris flows. Understanding the effects of wildfire on steep streams is integral to post-fire hazard assessment and mitigation of negative impacts to riverine ecosystems and downstream communities. In the Western Cascades, the historic magnitude and severity of the 2020 wildfires, coupled with variable physiography and timber harvest history, produces substantial uncertainty about post-fire fluvial geomorphic response. In this study, we evaluate the hydrogeomorphic response of steep streams in the Western Cascades to the 2020 Archie Creek and Holiday Farm fires, and assess the relative influence of biophysical variables (e.g., burn severity, precipitation, topography, forest age and cover) on morphologic change. We conducted channel surveys in December 2020 and June 2021 in five steep stream reaches spanning biophysical gradients to assess post-fire streamflow and sediment transport capacity, and changes in channel topography, grain size, and wood. Additionally, we completed uncrewed aerial vehicle surveys to create post-fire digital elevation models (DEMs) and, in turn, DEMs-of-difference maps using pre- and post-fire DEMs to measure topographic change from post-fire erosion and deposition. Preliminary observations from channel surveys show post-fire wood loading in all burnt study streams, one small landslide, and limited gravel aggradation. No new debris flows have been identified in our study areas, indicating that precipitation thresholds for debris flow initiation were not exceeded during the 2021 water year, or that root reinforcement provided enough stabilization on hillslopes to prevent mass wasting. Overall, initial geomorphic responses have been muted in our study streams, despite the high burn severity and steep topography of these sites, perhaps owing to below-normal precipitation in the winter and spring following the fires. Wood inputs to channels are likely to be the most persistent geomorphic effect in the first year after the fires. This work contributes to the greater understanding of landscape evolution following extreme fire.
Mentor Name
Andrew Wilcox
Hydrogeomorphic Response of Steep Streams after Wildfire in the Western Cascades, Oregon
UC 326
Severe fire may increase the vulnerability of steep mountain streams (5-10% slopes) to disturbance by increasing peak discharges, elevating inputs of sediment and wood into channels, and increasing the likelihood of landslides and debris flows. Understanding the effects of wildfire on steep streams is integral to post-fire hazard assessment and mitigation of negative impacts to riverine ecosystems and downstream communities. In the Western Cascades, the historic magnitude and severity of the 2020 wildfires, coupled with variable physiography and timber harvest history, produces substantial uncertainty about post-fire fluvial geomorphic response. In this study, we evaluate the hydrogeomorphic response of steep streams in the Western Cascades to the 2020 Archie Creek and Holiday Farm fires, and assess the relative influence of biophysical variables (e.g., burn severity, precipitation, topography, forest age and cover) on morphologic change. We conducted channel surveys in December 2020 and June 2021 in five steep stream reaches spanning biophysical gradients to assess post-fire streamflow and sediment transport capacity, and changes in channel topography, grain size, and wood. Additionally, we completed uncrewed aerial vehicle surveys to create post-fire digital elevation models (DEMs) and, in turn, DEMs-of-difference maps using pre- and post-fire DEMs to measure topographic change from post-fire erosion and deposition. Preliminary observations from channel surveys show post-fire wood loading in all burnt study streams, one small landslide, and limited gravel aggradation. No new debris flows have been identified in our study areas, indicating that precipitation thresholds for debris flow initiation were not exceeded during the 2021 water year, or that root reinforcement provided enough stabilization on hillslopes to prevent mass wasting. Overall, initial geomorphic responses have been muted in our study streams, despite the high burn severity and steep topography of these sites, perhaps owing to below-normal precipitation in the winter and spring following the fires. Wood inputs to channels are likely to be the most persistent geomorphic effect in the first year after the fires. This work contributes to the greater understanding of landscape evolution following extreme fire.