Inundation mechanisms and overbank water quality along the lower Mississippi River during the major floods of 2018, 2019, and 2020

• Flooding begins by seepage, then backwater, crevasse flow, and levee overtopping. • Overbank water quality was measured and sampled during the long-duration 2019 flood. • Nutrient concentrations decreased over the deeply inundated embanked floodplain. • Sand deposited on natural levees reduced overbank suspended sediment concentrations. • Most dissolved nutrients bypass through the embanked fluvial corridor. Artificially narrowed embanked floodplains, like those along the lower Mississippi River (LMR), are limited in their capacity to attenuate flooding and maximize nutrient retention. This study examines mechanisms and patterns of inundation during three major LMR floods (2018–2020) by analyzing water levels and temperatures from continuously operating sensors. Additionally, in-situ water quality measurements and overbank water samples collected in 2019 were analyzed for sediment, sorbed carbon and nutrients, and dissolved nutrients. Results indicate that subsurface seepage during the early rising limb initiates water-level increases in deep floodplain water bodies, followed by backwater inundation of distal floodplains, crevasse activation, and finally overtopping of natural levees near maximum stages. Overbank water samples from March and June 2019 indicate that dissolved nitrate-nitrite (54 % & 24 % less), phosphorus (27 % & 29 % less), and suspended-sediment concentrations (70 % & 59 % less) were lower than the LMR channel upstream, indicating nutrient retention and sediment deposition in the embanked floodplain; findings are partly supported by declining trends with distance from the channel bank. Conversely, mass percentages of sorbed carbon (235 % & 68 % more) and nitrogen (300 % & 125 % more) were higher than the LMR channel, resulting from sand deposition on natural levees and continued suspension of adsorptive fine-grained sediments. Subsequent fine-grained settling retains carbon and nitrogen in floodplain deposits. These findings establish that modes and patterns of sediment deposition and nutrient retention vary as stages adjust during long-duration floods, and while deeply inundated embanked floodplains remain capable of retention, fine sediment and nutrients primarily bypass downstream.