SUSPENDED-SEDIMENT AND WATER-QUALITY PATTERNS OVER A DEEPLY INUNDATED FLOODPLAIN ALONG THE LOWER MISSISSIPPI RIVER DURING THE 2019 FLOOD

A series of major floods occurred along the Lower Mississippi River (LMR) between 2018 and 2020, including the record-setting duration of the 2019 flood. In-situ water quality was measured and depth-integrated samples were collected at multiple sites over the floodplain downstream of Natchez, Mississippi, during major flood conditions in both March and June 2019, which was followed by sampling of overbank flood deposits in both 2019 and 2020. Cold water temperatures in March were associated with relatively high median values of dissolved oxygen (DO) (10.6 > 6.3 mg/L in June) and coarser suspended sediment grains (25.6 > 11 μm); whereas warm water temperatures in June supported relatively high median values of total dissolved solids (TDS) (243 > 169 mg/L in March), dissolved NO₂+NO₃ (1.21 > 0.46 mg/L), and dissolved phosphorus (P) (0.06 > 0.04 mg/L). Comparisons of overbank water quality and suspended sediment values to the LMR channel near Vicksburg indicate close correspondence of water temperature, DO in June, and TDS; whereas notable departures include lower median values of overbank DO in March (10.6 < 12.4 mg/L in LMR channel), pH (March ~6.9 < 7.6; June ~6.5 < 7.5), NO₂+NO₃ (March 0.46 < 1.00 mg/L; June 1.21 < 1.56 mg/L), P (March 0.04 < 0.06 mg/L; June 0.06 < 0.08 mg/L), and suspended sediment concentration (SSC) (March 44.2 < 146 mg/L in LMR channel; June 40.9 < 162 mg/L), and higher median percentages of adsorbed carbon (March 3.4% > 1.0% by mass; June 3.5% > 1.4%) and adsorbed nitrogen (March 0.4% > 0.1% by mass; June 0.4% > 0.2%). Some dissolved constituents (i.e., DO, TDS) are gradually depleted with distance across the floodplain; others remain unchanged (i.e., P). Overbank reductions in NO₂+NO₃ are not clear, but warmer water temperatures appear to promote sequestration. Overbank sedimentary characteristics are predominantly associated with sand deposited a short distance from the channel, leaving finer silts and clays to be flushed across the floodplain with adsorbed carbon and nitrogen. Despite considerable sediment bypass, natural levees accreted an average of 181 mm, meander scrolls accreted 30 mm, and backswamps 33 mm during the combined 2018 and 2019 floods; with an additional 101 mm along natural levees, 10 mm along meander scrolls, and 47 mm in backswamps during the 2020 flood.