Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Abstract

Many high-latitude fl uvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced baselevel fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profi le development. The Le Sueur River meanders as it incises, so we coupled the best-fi t incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fi ne sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fi ne sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased fl ows in rivers, leading to higher sediment loads, not just from fi eld erosion, but from increased bank and bluff erosion in the deeply incised valleys.