2024 CSDMS meeting-066


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Glacial isostatic adjustment drives spatiotemporal trends of meander migration rate in a former glacial lake basin

Samuel Kodama, UC Santa Cruz Santa Cruz California, United States. sakodama@ucsc.edu
Tamara Pico, UC Santa Cruz Santa Cruz California, United States. tpico@ucsc.edu
Noah Finnegan, UC Santa Cruz Santa Cruz California, United States. nfinnega@ucsc.edu
Mathieu Lapotre, Stanford University Stanford California, United States. mlapotre@stanford.edu
Jane Willenbring, Stanford Stanford California, United States. willenbring@stanford.edu
Ajay Limaye, University of Virginia Charlottesville Virginia, United States. ajay@virginia.edu

After Glacial Lake Agassiz drained ~8.5 ka, the Red River (North Dakota, USA) formed, flowing northward into Lake Winnipeg and incising into paleolacustrine sediment as it meandered. The Red River provides a natural experiment to interrogate the role of slope change on river meandering and morphologic evolution as it is characterized by shallow bed slopes (~0.0001), which have been controlled by crustal deformation due to glacial isostatic adjustment (GIA) in response to ice sheet unloading since the river’s inception. GIA has changed Red River channel elevation by 10s of meters, reducing slopes by up to 60% in the downstream reaches. We isolate the role of slope in order to explore its importance to lateral migration rate relative to other factors such as bank strength, sediment supply, and fluid flow.

We quantified the impact of GIA-induced slope changes on the Red River’s morphology by performing an analysis of river meanders and cutoffs (Kodama et al., 2023). We constructed a dataset that quantified the number of meanders, cutoffs, and modeled change in slopes caused by GIA along the Red River. Notably, the abundance of cutoffs normalized for channel width (a proxy for time-averaged meander rate) statistically significantly correlates with changes in slope, with far fewer cutoffs in the downstream reaches of the river, where the largest slope reduction occurred. We expanded this analysis to two tributaries of the Red River, and found that this relationship holds in all three river systems regardless of sign (negative or positive) of the GIA-induced slope change. We infer that slope drives changes in lateral migration rate for these detachment-limited systems by modulating the magnitude of shear stress on riverbanks.

We next developed a modeling framework by modifying a simple kinematic model of meander migration (Howard & Knutson, 1984) to explore the impact of GIA-induced slope change on the temporal trends of meander migration rate along the Red River. Previous work showed that the meander rate of two Red River meander scrolls exponentially decayed over the Holocene (Brooks 2003), which we are able to simulate with our GIA forced meandering model. Our study isolates the role of slope on river lateral migration and highlights how rivers near former ice sheets can respond to changes in slope that occur over thousands of years.