SEDIMENT IN THE SYSTEM: FINGERPRINTING SEDIMENT SOURCES IN THE MINNESOTA RIVER

Suspended sediment in the Minnesota River and tributaries exceeds EPA standards. Sediment deposition rates in Lake Peppin are currently 5-10 times pre-settlement rates, accelerating reservoir infilling to decades rather than centuries. Before mitigation programs can begin, the sediment source must be identified: does it come from topsoil erosion in the flat uplands, or from bluff erosion in the sharply incised rivers? The goal of the pilot project, funded by the Minnesota Pollution Control Agency, was to characterize the two end member sediment sources – upland and bluff – and to apply the results to a sediment core from a cutoff meander of the Minnesota River. Since the unique sediment characteristics were not known beforehand, we plan to apply a broad spectrum of techniques to the sediment, including fallout radionuclides, in-situ cosmogenic nuclides, major and minor element composition, sorbed nutrients, magnetic characterization, and mineral point-counts. The Fall 2005 Glacial Geology class performed the initial work on the pilot project, including field sampling and sample processing.

Group fieldwork included an overview drive through the uplands, a canoe trip along the bluffs, and initial lake coring setup. The class split into three groups to 1) sample upland till and lacustrian soils in the Beauford Watershed, 2) sample bluff tills along the Le Seur River (tributary to the Minnesota), 3) core Kasota Pond, a cutoff meander of the Minnesota River. Products from the soil group include compilation of a soils map in Arc GIS, detailed soil descriptions, textural analysis, percent organics (loss on ignition), and grain size analysis. The till bluff group produced sample descriptions, grain size analysis, and mineral point-counts. The lake core group produced a record of Minnesota flood stages, a core log, magnetic intensity log, smear samples at 10 cm intervals, organic content (loss on ignition), and preliminary 210Pb and 137Cs analysis. Interesting initial results include bimodal amounts of authigenic carbonate, which might indicate times of local pond deposition, punctuated by flood sediments. Ongoing work includes (Soils, S; Bluff, B; Lake core, L): fallout 210Pb and 137Cs (S, L): 210Pb and 137 analysis (S, L); in-situ cosmogenic nuclide 10Be:26Al ratios (S, B), major and minor element composition (S, B, L), sorbed nutrients (S, L), magnetic characterization (S, B), and mineral point-counts (S).