Aquatic Exploitation in the Lower Illinois River Valley: The Role of Paleoecological Change

Analyses of fish and freshwater mussel assemblages from the Koster site in the lower Illinois River valley provide corroborative data for independent geomorphic studies of floodplain evolution. Increases in riverine species of mussels occur at 7300 B.P. in the middle Middle Archaic at approximately the same time as the Illinois River ceased entrenching and began aggrading. Increases in quietwater mussels, bowfin, and bullheads starting around 5700 B.P. in the late Middle Archaic are coeval with the earliest date for Hartwell member lacustrine deposits and may herald the development of floodbasin lakes. These data support a two-phase model for enhancement of aquatic resources of the floodplain with potential improvement of river habitat occurring prior to emergence of productive floodbasin lakes. Although cultural factors certainly played roles in these shifts in the exploitation of aquatic taxa, similar patterns across broad areas suggest that the effect of Hypsithermal-related erosion on floodplain aggradation was an important factor.