WRS MS Defense

Controls on landscape evolution in post-glacial bedrock rivers, Lake Superior basin

River incision in bedrock landscapes is driven by uplift and base level fall, often producing knickpoints that propagate incision upstream. Incision is influenced by flow, sediment flux, and lithologic properties such as fractures, cleavage, and bedding planes. This study examines the role of base level fall and substrate erodibility, including crystalline and sedimentary bedrock and Quaternary sediments, in controlling the evolution of the post-glacial bedrock landscape of the north shore of Lake Superior. Three Lake Superior bedrock river watersheds, each with varying lithologic complexity and competency, were analyzed to investigate knickpoint formation, position, and migration rates, as well as the preservation of strath terraces formed by valley widening and incision. Strath terraces were mapped and dated to constrain the spatial and temporal evolution of longitudinal profiles following base level fall. Bedrock lithologies exposed in the channel were characterized using three erodibility metrics—uniaxial compressive strength, fracture intensity, and fracture density—and compared with channel hydraulic geometry and a stream power-based Erosion Index. Results indicate that bedrock competency exerts control on incision and planation at spatial scales ranging from individual reaches to entire profiles. Geologic complexity further influences profile evolution, with substrate transitions forming knickpoints and establishing local base levels. Three primary factors control bedrock river landscape evolution along the north shore: base level fall, basin geology, and drainage area. These factors drive the formation of knickpoints and strath terraces that provide insight into how bedrock landscapes respond to watershed-scale disturbances.