Post-fire debris flow survey of Big Creek

Hydrology Course Project Spring 2022

In this collaborative project for a hydrology course, two colleagues and I conducted field work, analysis and compiled a report of our findings from a creek one year after a post-fire debris flow.

Rendered facies maps of pebble counts I conducted.

Pebble count results

Details of the project are in the abstract below.

Though a natural disturbance in watersheds, wildfires are increasing in severity and increasingly affecting western US creeks. Post-fire debris flows can be triggered because of changed soil chemistry and loss of vegetation. The flood of sediment into the channel can result in large sediment deposits that may have implications for riparian species. We examined channel morphology and habitat for salmonids in Big Creek, California after a January 2021 debris flow. This debris flow and subsequent debris flood in Big Creek were triggered by intense precipitation in the Big Creek watershed, 97% of which had burned in the 2020 Dolan Fire. Researchers conducted a geomorphic survey of Big Creek in October 2021, establishing six cross-sections and conducting a longitudinal profile of the channel thalweg. In April 2022, we re-surveyed these cross-sections to document channel changes during high flows in December 2021, and we carried out pebble counts and facies mapping. We conducted a longitudinal thalweg profile of a lower reach that featured extensive terraces formed from debris flood deposits. We compared our data to the well-documented October survey and to a poorly- documented pre-debris flow survey to assess debris flood effects and subsequent channel recovery and habitat changes for the creek’s populations of steelhead and rainbow trout (Oncorhynchus mykiss). Our results show an increase in channel heterogeneity since the October survey, with the return of important geomorphic features (e.g. pools) and scouring of debris flood terraces. Comparison of the October and April cross-sections and longitudinal profiles showed a drop in the aggraded bed. We found suitable salmonid spawning habitat in some of the stream areas of our cross-sections based on the substrate composition of our surveyed facies. Our study shows the role of debris flows as key agents of morphological and ecological change in wildfire-prone landscapes.