LARGE FLOODS ON THE LOWER OHIO RIVER INFERRED FROM SLACKWATER DEPOSITS

Flooding causes millions of dollars of damage annually, but existing estimations of flood hazard are limited by the short timescale of instrumental data. Paleoflood hydrology is a valuable tool used to extend the instrumental record and reduce uncertainty in flood frequency analyses. Traditional approaches in paleoflood hydrology use slackwater deposits as flood event markers, though his approach has typically been limited to arid environments. However, recent work by Harden et al. (2021) found slackwater deposits along the Tennessee River, opening the door to robust reconstructions of flood discharges in the humid southeastern United States. Here we use a combination of hydraulic modeling, field sampling, and geochemical analyses to identify slackwater deposits along the lower Ohio River. A HEC-RAS model of the Ohio River was used to simulate river stages at a given discharge, evaluate inundation extent during past floods, and locate sites where slackwater deposits from large floods may be found. An initial survey conducted in June 2021 found evidence of a large flood event in the form a distinct sedimentary layer buried 15 cm in an alcove 12 m above the normal elevation of the river that likely corresponds to the 1937 flood of record on the lower Ohio River. We used X-ray diffraction to confirm that the mineralogy of the flood unit matches that of river load, and to distinguish it from the sediment that was produced by in-situ weathering of the limestone bedrock. ¹³⁷Cs and optically stimulated luminescence dating are being used to confirm the date of this deposit and provide a robust paleoflood chronology. Our initial survey did not identify additional slackwater deposits, but we plan to conduct another survey in fall 2021 to determine if there are prehistoric deposits that correspond to events that were larger than the 1937 flood. We anticipate that these slackwater deposits and resulting paleoflood record will be valuable for reducing uncertainty in flood frequency analysis on one of the largest commercial waterways in North America.