ABSTRACT

The most common overbank stratigraphic evidence of former large floods in meandering river systems of the Upper Mississippi Valley is textural reversals in fining-upward alluvial sequences. The overbank deposits of large floods can be grouped into coarse-grained (gravel) and fine-grained (sand, silt, and clay) deposits for purposes of paleoflood analyses. Graphical moments statistics have very limited usefulness for estimating relict flood magnitudes, but they are useful for differentiating whether overbank gravels were deposited from suspended load or bedload. The distinction is important because competent flow depths can be determined from the coarsest overbank particles transported as bedload. Relative changes in magnitudes of graphical moments statistics recorded across fine-grained overbank sediments at vertical increments of 1–3 cm show that deposits of large floods are more poorly sorted, less right skewed, and more platykurtic than adjacent deposits from small floods. A typical fine-grained depositional sequence for a large flood begins with high silt and low sand percentages in the basal section, progressively changes to high sand and low silt percentages in middle and upper middle sections, and culminates with a return to high silt and low sand percentages at the top of the sequence. Repetition of this sequential pattern across several geomorphic surfaces of different elevation is evidence of a very large flood, whereas variance between surfaces implies deposition from a small or moderate flood. Radiocarbon dated alluvial deposits indicate non-random recurrences for large floods during time scales spanning several millennia.