HR: 09:25h
AN: H31J-06    [Abstracts]
TI: Sediment provenance in the Minnesota River basin- Using lakes to determine river sediment source types
AU: * Blumentritt, D
EM: blum0123@umn.edu
AF: University Of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, United States
AU: Schottler, S
EM: schottler@smm.org
AF: St. Croix Watershed Research Station, 16910 152nd St. N, Marine on St. Croix, MN 55047, United States
AU: Engstrom, D
EM: dre@smm.org
AF: St. Croix Watershed Research Station, 16910 152nd St. N, Marine on St. Croix, MN 55047, United States
AB: The Minnesota River and its tributaries have suspended sediment loads that exceed EPA standards. Lake Pepin, a natural reservoir for the Minnesota River, Upper Mississippi River, and St. Croix River watersheds, currently has an accumulation rate of approximately 10 times pre-settlement rates and is the focus of restoration efforts. The Minnesota River watershed is >80% row-crop agriculture and has a significant amount of tile and ditch drainage networks. Although it only accounts for 38% of the Lake Pepin watershed, it contributes 80-90% of all sediment that reaches Lake Pepin. Before effective mitigation programs can begin in the watershed, sediment sources must be identified. The main sources for river sediments, as constrained by this study, are: (1) bank and bluff erosion of sharply incised stream channels and near-channel gully and ravine erosion at the bluff/upland interface, all of which are referred to as non-field sources, and (2) topsoil erosion from agricultural fields. Our goal is to apply a new sediment-tracing technique using the atmospherically deposited radionuclides ²¹⁰Pb and ¹³⁷Cs to apportion sediment to one of these original landscape positions. ²¹⁰Pb and ¹³⁷Cs are good fingerprinting radioisotopes because they are atmospherically deposited and are highly particle reactive, thus sediments from sources with significant atmospheric exposure, such as fields, have higher concentrations. Conversely, sediments from sources with minimal atmospheric exposure, such as banks, have negligible concentrations. The total amount of ²¹⁰Pb and ¹³⁷Cs in river sediments is determined by measuring concentrations on sediments deposited in backwaters and artificial impoundments. The signature of the radionucludes from field sources is determined by measuring flux in a series of reference lakes that have no significant channelized inputs. The reference lake system works on the assumption that the same processes that carry field sediment to lakes are the same in river systems. Sediment fingerprints are compared by using a simple mixing model to apportion sources. Preliminary results indicate that >70% of the total suspended sediments in the highly agricultural Minnesota River tributary watersheds are from near-channel, non-field sources. This agrees with the Lake Pepin sediment cores, which integrate the entire watershed, with 75% from non-field sources. Because the Lake Pepin sediment accumulation rate has increased ten-fold since European settlement in Minnesota and the non-field contribution is >70%, it can be inferred that the increase in suspended sediment is not a natural phenomenon.
DE: 1815 Erosion
DE: 1834 Human impacts
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2007 Fall Meeting