Alida Johnson

The rivers along Lake Superior’s North Shore have undergone significant geomorphic changes following glacial retreat. Ongoing baselevel change due to rapid drops in Lake Superior’s water level has compelled nearby rivers to evolve through fluvial erosion. Two erosive processes, abrasion and plucking, are known to control this evolution, yet their relative importance remains unclear. Abrasion wears down riverbeds through repeated collisions with fluvial sediments while plucking results from periods of high discharge breaking pieces off of fractured bedrock. This study aims to evaluate the impacts of these processes in shaping Devil Track River, a bedrock river found in northern Minnesota. Field methods for abrasion included Schmidt hammer measurements of bedrock compressive strength and grain size distribution analysis using Wolman pebble counts and the ImageGrains model. Plucking was investigated through fracture density with image analysis in the FracPaQ package within MatLab. Schmidt hammer results suggest that abrasion has played a limited role due to a weak positive relationship with slope. Additionally, low bed coverage and a lack of alluvial sections suggests a limited sedimentary presence and role for abrasion within Devil Track. The two grain size distribution methods employed revealed respective skews towards larger or smaller stones. High fracture density along the channel indicates that plucking is the dominant erosive process driving the river’s long-profile evolution. These results emphasize the importance of bedrock properties in determining which fluvial processes impel long profile evolution, and establish a guide for distinguishing between fluvial landscapes carved by abrasion against plucking in post-glacial landscapes.