LATE PLEISTOCENE AND HOLOCENE EVOLUTION OF THE UPPER MISSISSIPPI RIVER FLOODPLAIN

The upper Mississippi River (UMR) along Wisconsin's western border was a braided bedload river during the late Wisconsin glacial period 25,000 - 14,000 radiocarbon yrs B.P. During the transition to the Holocene a major reduction in inputs of coarse sediment to the UMR occurred because proglacial lakes along the margin of the retreating Laurentide Ice Sheet trapped much of the sand and gravel sediment load. Reduced bedload inputs and occurrences of large floods associated with lake outlet failures led to highly erosive discharges on the former steep-gradient floodplain. An age of 14,300 radiocarbon yrs B.P. (Beta 92064) (17,135 cal yrs B.P.) for a unit 4 m from the top of the late Wisconsin alluvium (Savanna Terrace surface) and an age of 13,545 radiocarbon yrs B.P. (AA 23384) (16,300 cal yrs B.P.) for the base of a former channel 37 m lower and associated with a flood-eroded, stair-stepped surface (Bagley Terrace complex) cut into the late Wisconsin alluvial fill is evidence that major incision occurred relatively quickly and removed massive amounts of sediment from the valley. The average depth of initial incision was about 16 m along most of the valley. At the termination of erosive meltwater from proglacial lakes, the thalweg of the UMR here had incised from 13-20 m below the level of the modern floodplain and from 30-50+ m below the maximum elevation of late Wisconsin alluviation. The relative depth of incision increases upstream. Incision of the UMR promoted active Holocene remobilization of sediment in tributaries that resulted in massive Holocene alluvial fans at many tributary mouths. The larger alluvial fans have maintained the UMR thalweg against the opposite valley side, and they have produced local convexity in the UMR long profile. Numerous radiocarbon ages from Holocene alluvium younger than about 7000 cal yr B.P. indicate rates of floodplain vertical accretion that average between about 0.04 and 0.10 cm/yr. These rates are between ¼ and ½ those for the entire Holocene and they underscore the massive early Holocene redistribution of sediment. The alluvial fans have promoted geographic stability of the UMR thalweg that, in turn, has led to long records of Holocene vertical alluviation underlying the modern floodplain in many areas of the UMR valley. This research was supported by the National Science Foundation (ATM-0112614).