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

Abstract

In bedrock landscapes, river incision is driven by uplift and base level fall, often generating knickpoints that migrate upstream as an incisional wave in response. The migration rate of knickpoints is influenced primarily by flow, sediment flux, and lithologic properties such as competence, fracturing, and bedding orientation. Strath terraces planated above the present river elevation provide spatial and temporal constraints on the incisional history of the longitudinal profile. The objective of this study is to explore the role of base level fall and substrate erodibility on fluvial landscape evolution in a post-glacial bedrock river setting by investigating the formation, position, and relative migration rates of knickpoints and the preservation of strath terraces in three young, bedrock rivers (Amity Creek, Lester River, and Mission Creek) on the north shore of Lake Superior. The Lake Superior basin experienced rapid lake level fall following the retreat of the Laurentide Ice Sheet, forming knickpoints in bedrock rivers that incise through a diverse range of Midcontinent Rift-aged crystalline and sedimentary bedrock, as well as Quaternary sediments. Strath terraces were mapped and dated to constrain the 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 three primary factors that control bedrock river landscape evolution along the north shore: base level fall following deglaciation; basin geology, specifically the geologic competency and complexity of the watershed; and drainage area upstream of knickpoints. Bedrock competency exerts control on incision from individual reaches to entire profiles. Channels incising through strong bedrock lithologies of the north shore are generally higher sloping and narrower, showing an adjustment in hydraulic geometry to account for substrate erodibility. Additionally, knickpoints are predominantly found in the strong lithologies. Variation in compressive strength measurements across surveys within a single lithology show that the weakest and most fractured surfaces are nearest to the knickpoint, as opposed to upstream and downstream, reflecting that weathering is centered around these transient points where incision rates are highest. Differences in profile form across the three watersheds are ultimately governed by the rate of surface weathering at the knickpoint: resistant lithologies sustain knickpoints longer due to slower weathering, while diffusion of these landforms is more rapid in the weaker lithologies. Competency also influences the planation of rivers, where variations in valley width and the distribution of strath terraces are attributed to the difference in relative erodibility between bedrock and Quaternary sediments. Geologic complexity further influences profile evolution, with substrate transitions forming knickpoints and establishing local base level controls. These transitions in substrate may stall headward knickpoint migration if a threshold drainage area is not reached based off the local hydrologic and sediment transport conditions. While disturbances like base level fall initiate transient responses in bedrock rivers, it is the basin’s unique geology that ultimately governs the formation and position of geomorphic landforms that offer insight into how bedrock landscapes respond to watershed-scale disturbances.