GEOARCHAEOLOGICAL EVIDENCE FOR LARGE HOLOCENE FLOODS IN THE SNY BOTTOM OF THE MISSISSIPPI RIVER VALLEY, WESTERN ILLINOIS

Landform sediment assemblages spanning the late Pleistocene through Holocene are well-represented in the Sny Bottom, where the Mississippi River occupies a long, broad (10 x 90 km) bedrock valley trench. Geological study of the many archaeological sites found along this stretch of the river reveals a stratigraphic record of very large (extra-historical?) floods with a recurrence interval of about 1,000 years. During flood stages the Mississippi metamorphoses from an island-braided channel to a larger, anastomosed system in which a complex hierarchy of secondary distributary systems is activated, a pattern that probably began ca. 7,000 yr B.P. Large floods complexly reorganize bottomland landscapes and ecological relationships, leaving a stratigraphic record of their occurrence and strongly influencing archaeological site formation processes in these settings. Prehistoric people were keenly adapted to flood events as evidenced by the representation of many kinds of floodplain sites, and by the rapid post-flood colonization of newly-made landforms.