QGG SHRODER MASS MOVEMENT AWARD: QUANTIFYING CHANGES IN CHANNEL MORPHOLOGY AND SALMONID HABITAT IN RESPONSE TO POST-FIRE DEBRIS FLOWS IN A STEEP, COASTAL STREAM, BIG SUR, CALIFORNIA

Debris flows are a common geomorphic response to wildfire in steep watersheds. Debris flows redistribute large volumes of sediment derived from hillslopes, channel beds, and channel margins and increase downstream sediment supply in fluvial systems. Channel morphology responds to sediment supply increases, and these changes influence physical habitat for aquatic species. While previous work explores the mechanics of post-fire debris flows and, separately, the impacts of elevated sediment supply on channel morphology, opportunities to examine the impact of post-fire debris flows on and reach- and bedform- scale channel morphology and physical habitat are rare due to a lack of high-resolution pre-fire baseline data. In August of 2020, the Dolan Fire struck California’s Big Sur coast, burning 125,000 acres of steep, coastal terrain. In January 2021, intense rainfall triggered a series of post-fire debris flows in the Big Creek watershed, a 57 km² coastal basin within the footprint of the Dolan Fire. Here, we present the results of pre- and post-wildfire structure from motion surveys and grain size analyses of two streams in the Big Creek watershed. Changes to the grain size distribution in Big Creek influence salmonid spawning habitat, as salmonid species require specific grain size conditions for successful spawning. We use fish length data and reach-averaged, remotely sensed grain size distributions to calculate movable fraction, a physical habitat index which represents the percentage of channel bed material that falls into a suitable size range for salmonid spawning gravels. We find that grain size decreased substantially following post-fire debris flows in Big Creek, resulting in substantial movable fraction increases. Wildfires are becoming increasingly common across the western United States due to anthropogenic climate change. As fire continues to exert a progressively greater impact on landscapes in the western United States, research examining the extensive geomorphic and ecological impacts of wildfire will become increasingly important.