Lower Mississippi River dynamics influence sediment and nutrient movement into the delta

The lower Mississippi River in Louisiana transports sediment and nutrients much differently than areas farther north, according to a new study.

Published in the Journal of Hydrology, the article outlines how the “backwater” areas of the lower river – an area where a river’s flow starts to slow down as it interacts with the ocean – impacts how sediments and nutrients eventually reach the Gulf of Mexico. Within the lower Mississippi River, the effects of this interaction between the river and Gulf of Mexico can be seen starting as far upriver as Vicksburg, Miss.

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.

“This is important because how sediment moves through the lowermost Mississippi reach in Louisiana is a primary control on things like dredging frequency, potential operations of sediment diversions to maximize capture of fine sediments, and a better understanding of how sand that is mined from the lower river bed for marsh creation and barrier island restoration is restored from upstream sediment input,” said Mead Allison, lead author of the report, the Institute’s Director of Physical Processes and Sediment Systems, and chair of Tulane’s Department of River-Coastal Science and Engineering. “What you see in this backwater area of the river is that the timing and amount of sediment being transported isn’t completely about how much is being supplied from upstream.”

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 the 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.

About The Water Institute of the Gulf: The Water Institute of the Gulf is a not-for-profit, independent research institute dedicated to advancing the understanding of coastal, deltaic, river and water resource systems, both within the Gulf Coast and around the world. This mission supports the practical application of innovative science and engineering, providing solutions that benefit society. For more information, visit www.thewaterinstitute.org.