Shifting sands: drivers and mobilization of fine sediment on the Cache la Poudre River following a wildfire
Date
2024
Authors
Katz, Aaron, author
Wohl, Ellen, advisor
Bestgen, Kevin, committee member
McGrath, Dan, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Cameron Peak wildfire (840 km2) of 2020 led to widespread but varied detrimental effects throughout the Poudre River watershed (4,895 km2). The Poudre River flows through a high gradient canyon section with a somewhat unimpaired flow regime before it reaches a low gradient transition zone with several human impacts including channel modifications and flow reduction. Burned tributaries contributed elevated levels of fine sediment (< 2 mm) to the mainstem Poudre within the canyon, and in 2021 a large debris flow in Black Hollow Creek, a canyon tributary, delivered substantial amounts of material, including fine sediment, directly into the mainstem Poudre River. This led to a major fish kill and the transportation and deposition of fine sediment for at least 100 km downstream. In the transition zone, extensive fine sediment deposits either partially blocked or filled several channel margin backwaters and side channels, which are important habitats for native fish, and fine sediment filled interstices of coarse substrate grains in the mainstem river, which impacts benthic macroinvertebrate and fish spawning habitat. I quantify the degree of fine sediment retention along 100 km of the Poudre River by measuring reach-averaged fine sediment volumes and embeddedness and use model selection of multiple linear regression models to determine whether distance downstream from the sediment source or reach-scale geometric variables are the primary drivers of fine sediment retention. I also conduct a flushing flows study using 2-dimensional hydraulic modelling to determine the discharge required to mobilize the substrate at four sites along the longitudinal gradient of the study area representing different geomorphic settings and hydrologic regimes. Results from model selection using Akaike's Information Criterion corrected for small sample size (AICc) show that for both metrics of fine sediment retention, reach location (canyon vs. transition zone) is the primary driver of sediment retention, but some reach-scale variables are significant predictors of fine sediment retention. Both fine sediment retention metrics (site-averaged volumes and embeddedness) are greater in the transition zone. At the reach scale, gradient and cross-sectional area are both significant predictors of embeddedness, and models with gradient as a predictor variable have substantial support in explaining site-averaged fine sediment volumes. A mixed model of embeddedness at the transect level with bedform as the fixed effect nested with site as the random effect indicates that fine sediment is preferentially retained in pools and that all backwaters are 100% embedded. Although there are only three sites upstream of Black Hollow, fine sediment retention is either greater or not statistically different than canyon sites downstream of Black Hollow. Hydraulic modelling of the 2-, 5-, and 10-year recurrence interval flows at four sites indicates major differences in the magnitude and frequency of bed substrate mobilization between the canyon and transition zone. At a high gradient canyon site, the 2-year flow mobilizes a substantial portion of the bed, while at transition zone sites, bed mobilization occurs only at the 5- or 10-year flow level. I posit that artificially reduced flows in the transition zone are responsible for the lack of bed mobilization and will lead to longer residence times of fine sediment and prolonged impacts to aquatic ecosystems. This study adds to the literature by investigating post-fire fluvial responses at a greater spatial scale than most previous studies of the matter. By quantifying spatial distribution, physical drivers, and mobilization potential of fine sediment following a large wildfire on a major river, we can better understand how large rivers with varied human impacts respond to major disturbances and make informed management and restoration decisions going forward.
Description
Zip file contains four images, an output CSV, and two transects spreadsheets.
Rights Access
Subject
flushing flows
hydraulic modelling
wildfire
fluvial geomorphology
fine sediment
sediment transport