Watershed Context for a Sediment Routing Model in the Le Sueur River, Southern Minnesota

Accurate prediction of sediment routing through fluvial networks is critical for theoretical models of landscape evolution, bedrock incision, and ecological habitat, as well as practical applications, such as estimates of load reduction in response to watershed management actions. Important aspects of sediment routing include identifying sediment sources, fluxes, transport pathways and grain size distributions, each of which vary over multiple spatial and temporal scales. We are combining mapping and granulometry with hydrologic data, high resolution aerial and ground-based LiDAR topographic data, historic air photo analysis, and geochemical fingerprinting to develop an integrated sediment budget and routing model for the Le Sueur River, southern Minnesota. The routing model will be used for evaluating management alternatives regarding source control for the purpose of reducing sediment yield to the Minnesota River. Here we report the results and methods for constraining several important components of the routing model including a) delineation of floodplain storage capacity from aerial LiDAR and field measurements; b) river meander migration rates over the past 70 years to constrain channel-floodplain exchange and determine locations where the river is prone to migrate more or less; c) the downstream evolution of alluvial grain size distributions as predicted by theoretical models and verified by detailed field measurements; and d) sediment contributions from steep, high-standing bluffs. The Le Sueur is a rapidly evolving system, with one large knickpoint migrating upstream at an average rate of 3 m per year over the past 11.5 kyr. In the wake of the knick, large ravines and bluffs are formed, fluvial terraces record the rate of vertical incision (5 mm/yr over the past 11.5 kyr) and meander migration rates systematically vary from 15 - 60 cm per year over the past 70 years.