Overbank sedimentation from the historic AD 2011 flood along the Lower Mississippi River, USA

In this study, we document sedimentary characteristics of overbank flood deposits associated with the epic A.D.2011 flood along the Lower Mississippi River (southern USA) and directly compare the findings to sedimentation from a comparable flood event in 1973, with the general purpose of understanding how extreme floods contribute to floodplain depositional patterns and accretion rates of embanked fluvial systems. The thicknesses of the 2011 flood deposits averaged 138 mm along natural levee crests, 9 mm on meander scrolls, and 3 mm in backswamps. These thicknesses are considerably less than those documented for the 1973 flood, sampled at the same locations. We contend that less sedimentation in 2011 occurred because the flood was not supplied with much upstream sediment from the Missouri River. Further, the 2011 sediments are coarser than in 1973, indicating that the higher 2011 flood levels were associated with more energetic overbank flows that flushed fine-grained sediments downstream within the narrow embanked floodplain corridor. The largest recorded flood in North American history is only marginally preserved in the embanked floodplain stratigraphy of the alluvial valley of Earth's third largest fluvial system.