Geomorphic and sedimentological controls on the effectiveness of an extreme flood

The 1993 flood on the Upper Mississippi River was a rare, large-magnitude hydrological event. Field and aerial survey analyses and Landsat 5 Thematic Mapper data were used to appraise the thickness of overbank deposits on leveed and unleveed reaches. Results indicate that minimal (<5 mm) overbank sedimentation occurred, except in the immediate vicinity of a levee break. Unleveed sections also lacked overbank sedimentation. Little geomorphological or sedimentological evidence of this extreme event is likely to be preserved. This raises questions about the completeness of the stratigraphic record: in situations where wide floodplains with cohesive soils provide effective resistance and dissipate energy so that erosion is minimized, and/or sediment supply is limited by event timing or sequencing, a large flood may leave little or no substantive evidence of its occurrence.