TERRACES OF THE EAST BRANCH CHIPPEWA RIVER, POPE AND SWIFT COUNTIES, MINNESOTA: A PROGLACIAL TRIBUTARY TO GLACIAL LAKE BENSON

During the latest Wisconsinan deglaciation the East Branch of the Chippewa River in west-central Minnesota was an outlet channels for the melting Des Moines Lobe. Terraces of the East Branch of the Chippewa River can be traced from the Swift Falls Delta Complex (a delta formed in Glacial Lake Benson) north to the Blue Mounds (a glacial landform on the Alexandria Moraine). The goal of this study was to: map the geomorphic evidence of the fluvial system, interpret the sediments exposed in a gravel pit located on the edge of the terrace and determine paleo-flow characteristics.

Terraces were mapped using topographic quadrangles and airphoto interpretation. Sedimentary structures were observed and interpreted in the field. Samples of the sediments were analyzed for provenance. Paleo-gradients were determined with cross sections and used to calculate paleo-flow and discharge rates

Three units were exposed in the gravel pit: till, fine sands, and a channel fill. Provenance analysis indicates that the Des Moines Lobe was the source of these sediments. The till appears to be ice rafted, this would indicate that the ice margin was close by. The varying clast sizes in the channel fill indicate fluctuating water velocities. On the basis of terrace gradients of 0.0017m/m, the average flow velocity was 3.8 to 6.3m/s and the average discharge was 38,000 to 63,000cu.m/sec

Terraces of the East Branch of the Chippewa River appear to be proglacial thus, at least a portion of the Des Moines Lobe was located on the Alexandria Moraine during the time Glacial Lake Benson existed.

Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 9820249).