SEDIMENTARY CHARACTERISTICS AND ASSOCIATED CARBON AND NUTRIENTS OF OVERBANK SEDIMENTS DEPOSITED DURING THE 2018, 2019, AND 2020 FLOODS IN EMBANKED FLOODPLAINS ALONG THE LOWER MISSISSIPPI RIVER NEAR NATCHEZ, MISSISSIPPI

The Lower Mississippi River (LMR) experienced major floods in 2018, 2019, and 2020. Sediment deposition in the embanked floodplains during floods represent important storage and sequestration opportunities for carbon and nutrients from ~40% of the continental USA. This research aims to compare depositional thicknesses, organic matter (OM), carbon (C), nitrogen (N), phosphorous (P) concentrations, and grain sizes in floodplain sediments deposited by the combined 2018-19 floods to the 2020 flood along the LMR near Natchez, Mississippi. Greater depositional thicknesses in 2018-19 are best explained by their combined flood durations; the 2019 flood was the longest in recorded history. Slightly higher levels of OM in 2018-19 sediments indicate preservation favored by the extended duration of saturated conditions and/or increased organic productivity in the warm overbank water column during the later summer interval of the 2019 flood. Comparable concentrations of total C and N indicate that post-flood organic productivity rapidly converts available C and N to biological forms while diverse soil compositions and redox conditions equilibrate the remaining soil C & N to predictable levels commensurate with the floodplain setting (i.e., natural levees, backswamps). Low concentrations of P indicate removal of P from the soil to the flood waters as conditions become anaerobic and P is reduced and dissolution of clay- bound phosphates. Grain size values are dependent on flood duration, suspended sediment concentration, and floodplain environment