LATE QUATERNARY DISCHARGE HISTORY AND GEOMORPHIC EVOLUTION OF THE UPPER MISSISSIPPI RIVER

The Wisconsin-stage Laurentide Ice Sheet advanced into the Mississippi River drainage basin, nearly doubling the Mississippi's drainage area by extending it to the ice divide in central Canada. At and near the time of maximum ice extent, the upper Mississippi River became a steep bedrock-bound flume that conveyed glacially-eroded sediments towards the Mississippi Embayment and Gulf of Mexico. The flickers of ice retreat and readvance during terminal Pleistocene warming cyclically routed water away from, and then back towards, the Mississippi River. Once large-scale ice retreat was underway, sediment-poor outflows from proglacial lakes incised the full-glacial-stage alluvial terraces, dropping base-level to Mississippi River tributaries and causing incision and delivery of alluvium to the mainstem valley. Full ice retreat from the Mississippi River drainage basin at 12.9 ka reduced the upper Mississippi River to an oversized headwaters gorge. Two final pulses of glacial lake meltwater flowed down the Mississippi until ~10.6 ka, when the upper Mississippi became a Holocene river with a discharge similar discharge that at present.

Oxygen isotope records from the Gulf of Mexico record the paleodischarge of the Mississippi River from 35-28 ka, 24-6.5 ka, and 1.4-0 ka. These records allow us to connect ice advance and retreat to the resultant meltwater routing, and can be transformed into a proxy paleohydrograph. LiDAR mapping of upper Mississippi River terraces, many of which are dated, ties fluvial response to past river discharge and outburst floods. Terrace elevations, slopes, and morphologies shed light on past sediment supply to the upper Mississippi valley and land-surface deformation generated by the Laurentide Ice Sheet forebulge. These together frame the interactions between changing sediment supply, water supply, climate, topography, and drainage basin area that shaped the upper Mississippi River and its tributaries over the last ~35,000 years.