OVERBANK SEDIMENT DEPOSITION VARIATION THROUGH HISTORICAL TIMEFRAMES IN HEAVILY MODIFIED FLOODPLAINS OF THE LOWER MISSISSIPPI RIVER – A SEDIMENTOLOGICAL AND GEOPHYSICAL APPROACH

Since the 1930s, the Lower Mississippi River (LMR) has experienced large-scale modifications to the channel profile and surrounding floodplains through dams, dikes, revetments, dredging, and channel cutoffs. Although these changes have improved navigation and reduced flood risk, unanticipated changes to the major flood return period, individual flood severity and duration, and sediment regime have become increasingly apparent and sometimes problematic, such as the 2011 and 2018-2020 floods. Flood control levees along the LMR have reduced the natural floodplain area by 70-90%, resulting in heavily restricted overbank storage capacity of water and sediment. For the same flood events in recent history, Natchez, MS, has experienced record high flood stages and much longer and more severe inundation compared to Vicksburg, MS. This research focuses on integration of clastic sedimentary characteristics and ground-penetrating radar (GPR) data from overbank profiles at Shipland WMA (48 km northwest of Vicksburg) and St. Catherine Creek NWR (18 km southwest of Natchez) using five 6-meter floodplain sediment cores from both locations in meander scroll and backswamp depositional environments. We seek to analyze any systematic depositional changes through time at both sites to determine whether large-scale river modification is distinctly reflected as changes through time in the overbank profile. This application of GPR in low-gradient, fine-grained meandering river floodplains has historically been underutilized, and this research seeks to test the capabilities of GPR in delineating wetland silt and clay facies along the LMR. Preliminary findings indicate correlative strata across each study site at various depths, possibly individual flood events, which will be further analyzed using grain size, GPR correlation, and C-14 time constraint to determine temporal and spatial variability in overbank deposition and flood processes between Shipland WMA and St. Catherine Creek NWR in the wake of a century of engineering and modifications.