The Kankakee Torrent and other large meltwater flooding events during the last deglaciation, Illinois, USA

Evidence of the Kankakee Torrent (Ekblaw and Athy, 1925) includes boulder bars formed on a scoured bedrock surface west of Kankakee, Illinois, and overflow channels that connect several moraine-dammed basins (Wauponsee, Watseka, and Pontiac; Willman and Payne, 1942). Geomorphic evidence of a large scale flood event in the Illinois Valley includes features such as erosional residuals (Hajic, 1990). Theageof the Kankakee Torrent is about 19,000calyrBP based on the pooled mean of four radiocarbonagesof tundra plant stems and leaves from the Oswego channel complex (median probability=18,930calyrBP, σ1 range, 18,870-18,970calyrBP). Analysis of recently obtained sediment cores from the middle Illinois River valley near Havana, Illinois, has revealed the bedrock surface is defended by a mantle of bouldery debris buried by 15m of mostly slackwater lake sediment. Radiocarbon ages of needles archived inthelake sediment reveal evidence for an early lake phase that post-dates the Kankakee Torrent (18,030-17,530calyrBP) and a later lake phase (15,690-13,040calyrBP). The radiocarbon ages indicate that the deeply buried bouldery rubble was deposited by the Kankakee Torrent. Consideration of isostasy indicates that the earlier lake phase at Havana may have been associated with downward flexure of land surface in response to glacier loading. The younger lake phase was caused in part by deposition of a sediment dam (the Savanna-Deer Plain terrace) at the mouth of the Illinois River. The lake shoaled due to passing of the isostatic forebulge across the area.