LATE GLACIAL AND HOLOCENE FLOODS, TERRACES, FLUVIAL LANDFORMS, EROSION AND SEDIMENTATION ON THE UPPER MISSISSIPPI RIVER

The upper Mississippi River (UMR) along western Wisconsin was still aggrading as late as about 17,100 cal yr B.P. (14,300 14C yr B.P.) as suggested by the age of an alluvial unit 4.3 m below the top of the maximum level of late Wisconsin alluviation. Many accept the idea that initial and deep incision of the late Wisconsin alluvial fill in the upper Mississippi Valley between Wisconsin and adjacent states of Minnesota and Iowa occurred in response to rapid drainage of Lake Agassiz after about 13,600 cal yr B.P. (11,700 14C yr B.P.) However, an age of 16,300 cal yr B.P. (13,545 14C yr B.P.) for peat in the bottom of a flood-carved channel incised 30+ m below the level of maximum late Wisconsin alluviation (Savanna Terrace surface) is about 16,300 cal yr B.P. (13,545 14C yr B.P.) at a site near Onalaska, Wisconsin. This age, if correct, indicates significant incision occurred prior to the Agassiz event. The likely cause is discharge from Glacial Lake Duluth via the St. Croix River. HEC-RAS model results for sites along western Wisconsin indicate the initial incision could have been associated with discharges as large as about 30,000 m3s-1, although a somewhat smaller magnitude is more likely. Subsequent large flows related mainly to discharge from Glacial Lakes Agassiz and Duluth resulted in further incision of the late Wisconsin alluvium to a depth of at least 52 m below the maximum level of late Wisconsin alluviation. The UMR and floodplain along western Wisconsin has experienced nearly continuous Holocene alluviation. Following an early Holocene episode of very rapid sedimentation and fan building at tributary mouths, the last 6000-5000 years have experienced a long-term average alluviation rate of about 0.09 cm yr-1. Human impacts have locally increased the natural rate of alluviation by an order of magnitude. This research was supported by the U.S. NSF (ATM-0112614).