Date of Award

Spring 5-2020

Degree Type

Masters Thesis

Degree Name

Master of Science (MS)


Biological, Environmental, and Earth Sciences

Committee Chair

Dr. Franklin T. Heitmuller

Committee Chair School

Biological, Environmental, and Earth Sciences

Committee Member 2

Dr. Davin Wallace

Committee Member 2 School

Ocean Science and Engineering

Committee Member 3

Dr. Paul F. Hudson

Committee Member 3 School

Biological, Environmental, and Earth Sciences

Committee Member 4

Dr. T. Markham Puckett

Committee Member 4 School

Biological, Environmental, and Earth Sciences


The Lower Mississippi River (LMR) near Natchez, Mississippi, 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 monitored the overbank flood pulses and associated sediment deposition along the LMR embanked floodplain during water years 2018 and 2019. Between March and May 2018, the LMR at Natchez flooded with a maximum stage of 17.41 m (57.12 ft), qualifying as the 5th 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, when the river exceeded flood stage through July resulting in a new flood duration record (212 days to date) and a maximum stage of 17.65 m (57.91 ft) on March 12th (third highest on record). Measured depths of the water column above the floodplain surface ranged from 2.60 to 7.75 m during the crest. Flow velocities measured at a 1.22 m depth across the floodplain ranged from 0.07 to 1.06 m/s.

Floodplain surface deposits were sampled in October 2017 and September 2018 before the onset of flood events; these were analyzed for grain size, organic matter (OM) content, magnetic susceptibility (MS), and total carbon (C) and adsorbed nutrients (N, P). Flood water and suspended sediment of the overbank water column were sampled during the 2019 flood (March and June), which included in situ measurements of temperature, pH, dissolved oxygen, and total dissolved solids. Suspended sediments in bottled samples were analyzed for concentration (SSC), grain size, carbon, and nutrients. Water in bottled samples was analyzed for dissolved phosphorus and nitrite-nitrate.

Overbank sediment properties in different hydrologic and geomorphological settings vary with proximity to the main channel. Floodplain surface and suspended sediments decrease in grain size (D50) as distance increases from the LMR channel. Generally, OM, C, N, and P increase with a decrease in grain size. An increase in suspended sediment concentrations and larger in grain size are directly correlated with an increase in overbank flow velocity and increased turbidity.

Using analysis of covariance (ANACOVA) (with D50 as the covariant), OM, C, and nutrients were statistically higher with distance from the LMR channel; OM, C, and nutrients were statistically higher in backswamps and swales relative to the other subenvironments. Subenvironment classifications (levee, backswamp, meander scroll) had a stronger correlation to grain size than grouping the data by year. When the data was grouped by year, little significance was found using ANACOVA.

Overbank water quality samples in 2019 were very similar among sampling sites during the same sample trip (i.e., same time). However, most parameters including temperature, pH, and dissolved carbon and nutrients were greater in June than March because of both seasonal influences and event hysteresis. Due to hysteresis peaks, suspended sediment concentration, nitrate plus nitrite, and phosphorus had peak values after the peak stage height, with no peak concentrations corresponding with maximum stage height. In both March and June, suspended sediment, phosphorus and nitrite-nitrate were lower than corresponding values in the LMR main channel.

Jansen_Costello_Thesis_11_30_2020.docx (70435 kB)
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Jansen_Costello_Thesis_11_30_2020.docx (70435 kB)
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