SEDIMENT CHARACTERISTICS OF AN EXTREME FLOOD - 1993 UPPER MISSISSIPPI RIVER VALLEY

The 1993 Mississippi River flood was notable for its high magnitude, long duration, summer occurrence, and low sediment discharge. A field survey of a 70-km-long reach in the vicinity of Quincy, Illinois, revealed that the event was characterized by <4 mm of vertical accretion on leveed and unleveed parts of the flood plain. Regional patterns of overbank suspended sediment transport and deposition were discerned from Landsat Thematic Mapper imagery. This >100 yr flood had remarkably little sedimentological or geomorphological impact on the flood plain within the study reach because the transport effectiveness of floods in large drainage basins is influenced by event sequencing in the same manner as floods in small watersheds, and the cohesive flood-plain soils were not susceptible to erosion.