Discharge controls on the sediment and dissolved nutrient transport flux of the lowermost Mississippi River: Implications for export to the ocean and for delta restoration

Researchers at The Water Institute of the Gulf, Tulane University, and the U.S. Army Corps of Engineers Engineering Research and Development Center used data from monitoring stations along the river and boat-based sampling – including following a parcel of water downriver from Vicksburg to the Gulf – to trace how water, sediment, and dissolved nitrate move through the lower river system at different river stages. Nitrate entering the river from fertilizer inputs and other sources in the drainage basin have been previously shown to be a fuel for the annual summer low oxygen “dead zone” in the Gulf of Mexico. The interaction of the reduced river water surface slope, tides, and penetration of saline water in a salt wedge creates a complex set of subenvironments that migrate upstream and downstream with river flow.

For the first time in a large river, the study confirms observationally the progressive energy loss through the backwater reach which acts to remove the heaviest sediment from suspension. It also shows how sediment can be stored seasonally on the river bed in the Louisiana reach, to be remobilized during the first rising flood of the year.

With respect to dissolved nitrate, the study found that at high flows, the concentrations in the river were the same in Baton Rouge as they were at the river’s exit points to the Gulf. However, at low flows, concentrations near the exits started to rise above the levels seen upriver – a result of additional nitrate flowing in from the Gulf in the salt wedge and then being mixed with the nitrate dissolved in the fresh river water coming downstream. During the first rising flood, when the sediment stored seasonally on the river bed is remobilized, a third source of nitrate from breakdown of organic matter in these deposits is released – further magnifying for a period of several weeks the nitrate delivered to the Gulf.

Results from the work are also designed to help calibrate numerical models simulating how the lowermost Mississippi River operates to aid in decision-making about future restoration and protection efforts along the river corridor. Link to the abstract here.