SEDIMENTARY DYNAMICS AND WATER QUALITY IN FLOODPLAINS OF THE LOWER MISSISSIPPI RIVER NEAR NATCHEZ, MISSISSIPPI, DURING THE 2018 AND 2019 FLOODS

The Lower Mississippi River (LMR) near Natchez, Mississippi, has experienced intervals of major flooding during the 2018 and 2019 water years. While federal agencies sample sediment and water quality in the main LMR channel, little is known about sedimentary dynamics, water quality, and nutrient concentrations of the overbank water column within the embanked LMR floodplain. These data are needed to support ecological floodplain restoration efforts and provide context for downstream concerns including sediment delivery to coastal zones and hypoxia in the Gulf of Mexico.

This study contextualizes suspended sediments, carbon, and nutrients (N, P) of the overbank water column during the 2019 major flood (sampled in March and June) with floodplain deposits sampled in October 2017 and September 2018 before the onset of flood events. Between March and May 2018, the LMR at Natchez flooded with a maximum stage of 17.41 m (57.12 ft), qualifying as the 4^th highest crest on record. The river receded but remained above normal stages through September when another rise pushed the river above action stage through January 2019. Since January 2019, the LMR has remained above flood stage resulting in a new flood duration record (172 days to date) and a maximum stage of 17.65 m (57.91 ft) in March, the 3^rd highest crest on record. Measured depths of the water column above the floodplain surface ranged from 4.82 m (meander scroll ridge) to 7.74 m (backswamp) during the crest. Flow velocities measured at a 1.2 m depth across the floodplain ranged from 0.07–0.62 m/s.

Preliminary results for particle size of floodplain deposits sampled before flooding include a decrease in median size (d50) from natural levees (0.159–0.189 mm) to backswamps (0.018–0.109 mm). For meander scroll areas, d50 ranges from 0.092–0.116 mm and 0.025–0.058 mm for ridges and swales, respectively. Preliminary results (excluding nutrients) for selected water-quality parameters measured at 0.25 m depth in March 2019 include averages for dissolved oxygen (DO) of 90.4%, pH of 7.1, and total dissolved solids (TDS) of 169 mg/L; and depth-integrated suspended sediment concentrations (SSC) across the floodplain range from 34.6-53.0 mg/L. These compare to 101% DO, 7.6 pH, 138 mg/L TDS, and 146 mg/L SSC measured in the main channel upstream at Vicksburg during the same 3-day timespan.