Date of Award:

12-2023

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Patrick Belmont (Co-Chair), Brendan Murphy (Co-Chair)

Committee

Patrick Belmont

Committee

Brendan Murphy

Committee

Larissa Yocom

Committee

Jon Czuba

Abstract

Flooding and sedimentation caused by wildfire are among the greatest threats to watersheds, fish populations and reservoirs in the western US. Burned landscapes are at risk for increased runoff and erosion and have the potential to transport sediment that may put downstream resources at risk. The ability of the channel to transport sediment downstream, known as the connectivity, is important for determining where impacts may occur. Sediment bottlenecks are locations within the watershed where local conditions produce a persistent decrease in downstream connectivity of sediment, resulting in increased sediment deposition and potentially a substantial modification of the local channel and floodplain. The primary objective of this research is to evaluate the volume, location, and amount of sediment bottlenecks in watersheds after wildfire. We identified and surveyed 86 sediment bottlenecks associated with 15 wildfires throughout Utah. The mechanisms responsible for these sediment bottlenecks were attributed to either the geometry of the channel and floodplain or physical obstructions, including large in-stream wood, beaver dams, debris flow deposits, and human infrastructure. We measured channel/floodplain geometry and land cover characteristics using GIS, which we then compared to the volume and location of these sediment bottlenecks. Additionally, we drew large wood and debris flow deposits in GIS at each site from aerial imagery, as both often increase considerably after wildfire and can significantly influence the amount of sediment bottlenecks. Our results indicate that the geometry of the channel and floodplain influences sediment bottlenecks in burned watersheds, directly causing sediment deposition in some locations and influencing occurrence of other physical obstructions on sediment deposition in other locations. Beyond local controls, several watershed attributes exert a significant influence on the recruitment and transport of sediment and wood. These findings will help refine sediment routing models, assist in identifying the magnitude and location of potential sedimentation risks, and better inform the management of infrastructure and aquatic habitat after wildfire.

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