GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION KIRK BRYAN AWARD: LANDSCAPE EVOLUTION, VALLEY EXCAVATION, AND TERRACE DEVELOPMENT FOLLOWING ABRUPT POST-GLACIAL BASE-LEVEL FALL

Continental glaciation left its imprint on many rivers in high latitude post-glacial landscapes. Channels that experienced base level fall are still incising through glacial tills, which can be competent enough to behave like weak bedrock, yet weak enough to erode rapidly. Because of these properties, valley excavation in till-based post-glacial channels provides a unique opportunity to study river incision and knickpoint migration in transient systems. In the Le Sueur River in central Minnesota, USA, we mapped hundreds of strath terraces formed from planation of glacial tills during river response to incision of the Minnesota River valley at the end of the last glaciation. Optically-stimulated luminescence and radiocarbon dating were used to date fluvial deposits on strath surfaces at varying heights and distances upstream. These data were combined with numerical modeling to determine knickpoint migration and incision history. Results show the Le Sueur is best modeled as a detachment-limited channel, with downstream coarsening playing a critical role in accurately modeling longitudinal profile evolution.

In addition to providing insight into the mechanics of knickpoint migration in transient systems, combining channel evolution modeling with a detailed modern sediment budget allowed us to study the impacts of Euro-American settlement on sediment loading. The Le Sueur River today has high sediment loads, with most sediment coming from erosion of high bluffs in the deeply-incised knickzone. We coupled the best-fit incision model to a meandering model to see how sediment loads evolved over time, thus allowing us to determine the natural background sediment load prior to agricultural expansion in the mid-1800s. We found that modern fine sediment loads associated with valley excavation were three times higher than pre-settlement loads while overall modern fine sediment loads were 4-5 times higher. Recent changes in hydrology associated with land use and climate change have increased sediment loads in rivers, not just through increased field erosion, but also by increasing bluff and bank erosion in deeply-incised valleys.