Late Glacial Landscape Evolution of the Upper Mississippi River Valley and its Tributaries

James E. Sullivan

Glaciofluvial terraces in the Upper Mississippi River Valley (UMRV) between southeast Minnesota and western Wisconsin reflect episodes of infilling and downcutting that coincide with the complex multiple glacial advance and retreat during the Late Pleistocene, 16,000 to 10,000 BP. The tributaries of the UMRV responded to the base-level changes with corresponding changes in grade, length and basin characteristics.

Using air photos, topographic and surficial geologic maps, outwash aprons of the Superior Lobe and Grantsburg Sublobe of the Des Moine Lobe were identified and traced to their respective ice margins. Terraces were identified and correlated to their parental ice lobes based on sediment composition. Stream profiles of the tributaries reveal a number of knickpoints that relate to base-level changes of the UMRV. Mathematical relationships between watershed area and average sediment size was used to determine former slopes of the tributaries during base-level changes.

During the St. Croix Phase, 16,000 BP, the Superior Lobe invaded east central Minnesota, occupying the UMRV from the Twin Cities area northward. Outwash of the Superior Lobe filled the UMRV to approximately the 245 meter elevation at the ice margin. Between 16,000 and 14,000 BP, sediment depleted water from glacial lakes and runoff downcut the UMRV and tributary floodplains. The Grantsburg Sublobe of the Des Moine Lobe advanced 14,000 BP, filling the UMRV with outwash to the 220 meter elevation. Contemporaneously, the St. Croix River carried glacial outwash of the Superior Lobe into the UMRV.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995