A COMPARISON OF OVERBANK SEDIMENTATION THICKNESS AND TEXTURE DURING THE 2011 AND 1973 FLOODS IN NON-EMBANKED FLOODPLAINS ALONG THE LOWER MISSISSIPPI RIVER, USA

The 2011 flood along the Lower Mississippi River established new stage records at various locations from Mississippi (peak discharge of 67,394 m³/s at Natchez, Mississippi) to Louisiana, a segment that includes the Old River Control Structure, which prevents a massive, western avulsion to the Atchafalaya River. This study specifically documents depositional thickness and texture of overbank sediments associated with the 2011 flood at 55 sites between Natchez, Mississippi and St. Francisville, Louisiana, and compares the results with floodplain deposition during the prolific 1973 flood in the same area. Notably, the study sites occur in floodplains that are not protected by flood-control levees (dikes). Also, the study reach has exhibited an alarming trend of increasing stage for a given flood discharge. Results show considerable variability in sediment thickness, ranging from <1 mm to 620 mm with the thickest deposits along natural levees. At some sites on natural levees, however, little sediment was deposited despite being inundated to a depth of about 4 m. Overall, most sites are characterized by less than 10 mm of overbank sediment and are considerably less than thicknesses measured following the 1973 flood. All depositional sub-environments, including natural levees, meander scrolls (point bars), and backswamps, were subject to less sedimentation than the 1973 flood. Additionally, comparisons of texture with the 1973 flood data indicate that natural levee deposits are slightly coarser, minimally thick deposits on meander scrolls are finer, and backswamp deposits are somewhat coarser for the 2011 flood sediments. These data indicate relatively energetic, sediment-deprived overbank flow conditions that possibly entrained pre-existing surficial floodplain sediments from proximal sub-environments and transported them to distal zones of the floodplain. The well-documented decline of suspended-sediment loads along the Lower Mississippi River facilitates explanation for the unimpressive sedimentation in 2011, but another causative factor is that the annual peak suspended sediment load (1,046,000 tons/day) for 2011 occurred during a minor overbank event two months prior to the larger flood, whose peak discharge was associated with a suspended sediment load of only 727,400 tons/day.