Changes in sediment characteristics in step-pool streams after wildfire

Authors: Hannah Gnoza*, University of Colorado Denver, Anne Chin, University of Colorado Denver, Anna P. Solverson, University of Colorado Denver, Samantha M. Sellers, University of Colorado Denver, Linda S. O'Hirok, California State University Channel Islands
Topics: Geomorphology
Keywords: wildfire, geomorphology, ecology, climate change
Session Type: Poster
Day: 4/5/2019
Start / End Time: 8:00 AM / 9:40 AM
Room: Lincoln 2, Marriott, Exhibition Level
Presentation File: No File Uploaded

Steps and pools are stable features in mountain streams, providing energy dissipation in high-energy environments. Under climate change, these bedforms are susceptible to de-stabilization by intensifying disturbances such as wildfires and floods. When wildfire occurs, vegetation loss decreases organic matter and soil cohesion, leading to soil water repellency and decreases in infiltration. These changes increase runoff and flooding potential. Barren hillslopes also enhance erosion and sediment input into river channels. Wildfires therefore typically increase water and sediment hazards and degrade ecosystems, impacting human communities downstream. The 2012 Waldo Canyon Fire in Colorado, USA, offered opportunity to document biogeomorphic responses of step-pool streams after wildfire. Seven study reaches in Pike National Forest burned by a range of severity were compared with three unburned reference reaches. Over three post-fire seasons, topographic surveys and light detection and ranging terrestrial laser scanning documented erosion and deposition within the study channels. Collection and analysis of benthic macroinvertebrates similarly provided insights into ecological responses. Building upon the larger study, this presentation reports changes in sediment characteristics within study channels affected by the fire. Volumetric samples were collected, sieved, and analyzed for size distributions through the post-fire seasons. The Wolman pebble count provided data for study reaches with coarser materials. Results show fining in study reaches where substantial deposition of sediment buried step-pool structures. Fining also occurred in study reaches where floods dislodged large step clasts. These data fill gaps and enhance understanding of the detailed processes of erosion and sediment movement after wildfire in step-pool steams.

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