Holocene Mississippi River floods and geoarchaeological site formation processes in the Sny Bottom, western Illinois, USA

Holocene alluvial deposits are well represented in the Sny Bottom, a reach of the Mississippi River in western Illinois characterized by the presence of a long anabranch channel (the Sny). Prior to its engineered confinement, the flood-stage Mississippi functioned as an anastomosed system comprised of many secondary landscape elements hierarchically arranged to distribute water out across a broad floodplain. Radiocarbon-dated stratigraphic sequences are used to reconstruct the geological history of this river reach as it metamorphosed from a sandy Late Pleistocene braided floodplain into a system dominated by mud. By 7400 cal BP, the Mississippi migrated to its present position and began to build the natural levee it would use for the remainder of the Holocene. Major subsequent geomorphological adjustments to the system resulted from large floods at ca. 6900, 4900, 3300 and 2000 cal BP. Valley fill is a complex arrangement of individual sedimentary bodies that accumulated at highly variable rates. Vertical accretion by sedimentation and soil upbuilding acted together to preserve numerous buried archaeological sites of all ages.