Landscape evolution, alluvial architecture, environmental history, and the archaeological record of the Upper Mississippi River Valley

The alluvial fill in the Upper Mississippi River Valley (UMV) is a palimpsest of past landscapes, environments, and physical evidence of human life ways. The valley has undergone significant changes in fluvial style during the time humans have occupied its landscapes, including changes in channel pattern, location of depocenters, and sediment lithology. Remnants of late-glacial braidplains that predate human presence in the Upper Midwest occur as sandy terraces and terrace fills. A subsequent major change in alluvial architecture resulted from fundamental changes in seasonal water and sediment input that marked the end of glacial meltwater input into the valley about 12.4 ka (10,500 14C yr B.P.). A shift to net transport of sandy bedload and storage of fine-grained overbank sediment on the floodplain accompanied the change to an island-braided channel pattern at that time. The Holocene channel belt has been significantly narrower, and the zone of sediment storage is reduced relative to that of the late-glacial river. Major Holocene depocenters include alluvial fans and colluvial slopes, floodbasins, natural levees, and fluvial fans at the junction of large tributaries.Climate models, and paleobotanical, and isotopic studies indicate that shifts in large-scale patterns of atmospheric circulation and moisture transport into the mid-continent of North America induced hydrologic and vegetation changes that strongly influenced flood frequency and magnitude, the delivery of sediment from tributary basins, and the evolution of the UMV landscape. Understanding the alluvial architecture of the valley, and the temporal/spatial distribution of biotic environments and processes that have buried, mixed, altered, or destroyed archaeological deposits is essential to develop strategies for sampling the valley for evidence of past human activity and for properly interpreting the archaeological record.