Timing and pattern of valley excavation, Le Sueur River, south-central Minnesota, USA

Abstract

Strath terraces of the Le Sueur River, south central Minnesota, preserve the record of river incision. A combination of airborne LiDAR (light detection and ranging) and terrace dating through optically-stimulated luminescence (OSL) and radiocarbon methods were used to construct a conceptual model of valley excavation during the Holocene and late Pleistocene from lateral and vertical incision. The river is responding to approximately 70 meters of base level fall that occurred 13,400 years ago (11,500 radiocarbon years before present), when glacial River Warren carved the Minnesota River valley. The carving of the Minnesota River valley led to widespread incision on Minnesota River tributaries as knickpoints propagated upstream from the main stem Minnesota River. As the knickpoint moved up the Le Sueur River, hundreds of terrace surfaces were formed. These terraces are strath terraces carved into glacial tills, with alluvial deposits overlying planed-off till surfaces. Observations from dating terrace alluvium indicate that the river underwent relatively continuous incision, which is ongoing today. The incision model derived from terraces ages was coupled with valley geometry measured from LiDAR data to determine how valley excavation rates have changed through time. Results from this conceptual model indicate that valley excavation has been relatively constant through time. When this background valley excavation rate was compared with the modern sediment load, it was determined that the modern sediment load is 4-5 times greater than the average Holocene sediment load. This demonstrates that the post-settlement load is greater than the pre-settlement load and should guide the management of this basin to focus on the anthropogenic changes to the basin.