IMPACT OF HOLOCENE VALLEY EVOLUTION ON MODERN SEDIMENT LOADING IN THE LE SUEUR RIVER, MINNESOTA

Terraces in the Le Sueur River in Minnesota record the evolution of valley incision and knick point migration following the catastrophic drainage of Lake Agassiz through its southern outlet ~11,500 rcbp. We are analyzing terrace history to determine erosion rates through the Holocene both to understand how valleys evolve following 70 meters of sudden base level fall and to determine pre-settlement erosion rates in the lower Le Sueur River valley. The Le Sueur River currently carries a high sediment load, far exceeding water quality standards, and there is a need to better understand natural background sediment loads. Holocene erosion rates are compared to a modern sediment budget we are constructing for the Le Sueur that compares bluff, ravine, streambank, and upland loads both above and below the knick zone. Modern sediment loads from paired gages show a disproportional amount of the total sediment comes from the deeply-incised valley below the knick zone where bluff and ravine erosion becomes increasingly important.

Over 500 terraces were mapped using 1-meter resolution LiDAR data and grouped into terrace sets based on topographic relationships. Terraces were dated using a combination of radiocarbon and optically-stimulated luminesence (OSL) techniques. Coupling of topography, depositional age, and properties of the underlying tills show that erosion rates (and thus sediment loads) were high initially and decreased by a factor of three during the mid-Holocene dry period. Pre-settlement sediment loads were lower than modern loads by more than can be accounted for through increased upland erosion alone. Bluff erosion rates determined from ground-based LiDAR scans and aerial photographs show that erosion of high bluffs dominates sediment loads in the valley below the knick zone highlighting the importance of Holocene incision history and valley evolution on modern sediment loading.