FLOODS, SEDIMENTATION, AND OTHER ALLUVIAL ACTIVITY ON THE UPPER MISSISSIPPI RIVER DURING THE HOLOCENE

The Holocene upper Mississippi River (UMR) before about 7000 cal yrs B.P. was strongly influenced by tributary sediment remobilization responding to late-glacial incision of the main channel UMR. Since about 7000 cal yrs B.P. flood episodes related to climate changes best explain alluvial activity on the UMR. Following retreat of glacial ice from the basin headwaters about 16,800 cal yrs B.P., the late Wisconsin alluvial fill was deeply incised by highly erosive waters from proglacial lakes and by extreme floods associated with outlet failures and rapid drainage of proglacial lakes. This incision accelerated sediment removal from side valley tributaries as a result of local baselevel lowering and led to alluvial fan building in the main UMR valley. Fan building slowed by about 7000-6000 cal yrs B.P., after which, climatic change and its influence on floods became the dominant force controlling UMR alluvial activity. Anthropogenic forces significantly influenced small to moderate, high frequency fluvial activity beginning about 150-200 years ago. A major ecotone within the UMR basin separates forest to the northeast from prairie to the southwest, and its association with relatively steep seasonal air mass boundaries accounts for the UMR basin's high sensitivity to changes in atmospheric circulation regimes. The largest late Holocene floods were associated with relatively cool/moist climate episodes, whereas relatively warm climate episodes were particularly characterized by extreme hydrologic variability with frequent very low flows and near absence of floods in some years being separated by relatively frequent occurrences of large floods in other years as occurred between about 5000-3000 and 1000-600 cal yrs B.P. During the last 3000 years, exceptionally large floods occurred at about 2700-2500, 2200, 1100-900, and about 600 cal yrs B.P. These large floods resulted either from snowmelt, probably combined with widespread rainfall, and/or from prolonged heavy summer rains associated with stalled cold fronts, but not from changes in land cover. The largest late Holocene floods commonly occurred during times of rapidly changing climate. This research was supported by the U.S. NSF (ATM-0112614).