Mechanics of River Erosion and its Effects on Floodplain Biogeochemistry

• Main Author: Douglas, Madison Marie
• Format: Thesis
• Language: English
• Published: 2023
• Subjects: Arctic; Climate change; Ice; permafrost; Alaska
• Online Access: https://thesis.library.caltech.edu/15263/; https://thesis.library.caltech.edu/15263/13/MDouglas_Thesis_Rev2.pdf; https://thesis.library.caltech.edu/15263/8/Movie_3_S1_Experiment1.mp4; https://thesis.library.caltech.edu/15263/7/Movie_3_S2_Experiment2.mp4; https://thesis.library.caltech.edu/15263/6/Movie_3_S3_Experiment3.mp4; https://thesis.library.caltech.edu/15263/5/Movie_3_S4_Experiment4.mp4; https://thesis.library.caltech.edu/15263/4/Movie_3_S5_Experiment5.mp4; https://thesis.library.caltech.edu/15263/9/8_SM1.mp4; https://thesis.library.caltech.edu/15263/10/8_SM2.mp4; https://thesis.library.caltech.edu/15263/11/8_SM3.mp4; https://thesis.library.caltech.edu/15263/3/SupplementalTableS9_1.csv; https://resolver.caltech.edu/CaltechTHESIS:06012023-210614776

Rivers transport water, sediment, and nutrients across Earth’s surface. They shape landscapes, eroding mountain ranges and building floodplains, simultaneously providing important resources and posing a hazard to nearby communities. Here, I present field work, flume experiments, numerical models, and laboratory analyses addressing three main themes: permafrost river and floodplain dynamics, river meandering without plants, and rates of bedrock incision. Arctic rivers migrate rapidly across their floodplains and their migration rates are predicted to increase as permafrost thaws due to climate change. However, no mechanistic model is capable of predicting permafrost riverbank annual erosion rates. To address this knowledge gap, I developed a calibrated numerical model for permafrost riverbank erosion. A previously published theory assumes that permafrost erosion rates are limited by pore-ice thaw, but underestimates thaw rates due to bank roughness increasing heat transfer from the river to its banks (Chapter 3). Results indicate that thaw-limited erosion is orders of magnitude higher than observed erosion rates, and permafrost riverbank erosion must instead be limited by sediment entrainment and the collapse of overhanging blocks to match observed rates (Chapter 2). Based on experimental results, I developed a 1D numerical model that includes roughness-dependent permafrost thaw and sediment entrainment and tracks how heat transfer within the riverbank can form a thawed layer (Chapter 4). Results indicate that permafrost riverbank erosion rates respond to changes in river discharge due to climate change, which affect both bank thaw and entrainment rates, and are only sensitive to changes in water temperature via thawed layer failure. As a case study, I conducted fieldwork along the Koyukuk River in Alaska, which is located in discontinuous permafrost. I found that changes in riverbank erosion rates may more rapidly erase permafrost from floodplains (Chapter 7) and change the spatial patterns of floodplain methane ...