HOW FAR CAN YOU EXPAND YOUR SEDIMENT BUDGET? ATTEMPTS TO TAKE AN INTEGRATED SEDIMENT BUDGET AND EXTRAPOLATE IT TO NEIGHBORING BASINS IN AN AGRICULTURAL WATERSHED IN THE UPPER MIDWEST, USA (Invited Presentation)

Excess fine sediment is a common problem in agricultural basins. In incising systems, small areas may contribute the vast majority of the sediment, and thus water quality management requires a strong understanding of sediment sources and sinks to target the right locations within the basin. To aid with management efforts, we constructed a sediment budget within the 2880 km² Le Sueur basin in Minnesota, USA, taking advantage of all available data: historical air photos, repeat terrestrial laser scans, geochemical fingerprinting, in-channel surveys, analyses of aerial lidar, event-scale monitoring in ravines, and an extensive stream gaging network. Given the complexity of sediment budgeting and large array of data involved, the question was raised: Can this budget be applied elsewhere? To test the portability of the Le Sueur budget, we created sediment budgets for two neighboring basins with similar geomorphic environments in which extents of the four major sediment sources (bluffs, streambanks, ravines, and upland fields) were measured directly, but erosion rates were initially derived from the Le Sueur. Budget predictions were compared with gaging records and total suspended solid (TSS) loads, adding increasing amounts of locally-derived erosion data. Much of the effort focused on bluffs, the dominant sediment source. For bluff erosion, we found efforts were best spent getting an accurate inventory of extents rather than measuring all possible bluff retreat rates. Bluff retreat rates were best averaged at the subbasin scale, with subbasins delineated based on geomorphic environment. Sediment fingerprinting using ²¹⁰Pb and meteoric ¹⁰Be was used both to determine upland sediment yields by coupling fingerprinting with upland gage TSS data and as a check on the final budget using data collected at river mouths. Challenges encountered included A) lack of upland TSS data in neighboring Blue Earth and Watonwan basins, B) more ravine heterogeneity in the Watonwan basin, and C) over-estimation of sediment loads overall in the Watonwan basin. The sediment budget has enhanced a network routing model with spatially-explicit sediment inputs and allowed development of a management options simulation model to investigate a variety of management strategies to reduce sediment loading in these large agricultural basins.