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Fresh water from Bonnet Carré Spillway has hurt the Gulf Coast. Could upriver diversions help?

The Bonnet Carre Spillway, just before an opening in March 2018.
Travis Lux
The Bonnet Carre Spillway, just before an opening in March 2018.

Punching two more channels upriver from the Bonnet Carré Spillway could bolster habitat nearby while lessening the environmental harm caused farther south, according to a recent Tulane University study.

Within the past six years, opening the spillway’s gates has become an almost annual affair. During the historic flood of 2019, part of the Mississippi River’s flow was redirected not once, but twice to protect more than one million people living in New Orleans, St. Bernard and Plaquemines parishes.

However, opening the gates diverts the fresh, nutrient-rich river water into Lake Pontchartrain and out into the waters around the Chandeleur Sound and the Mississippi coast with few wetlands in between. It disrupts the local ecosystem, scattering salt-loving shrimp, causing algal blooms and even giving dolphins painful skin lesions due to the rapid and persistent influx of freshwater.

Tulane’s team of scientists as well as state coastal officials are hopeful that two proposed river diversions – Union and Ama – could improve how the river is managed and reintroduce the Mississippi’s water where it’s needed.

Unlike the $2 billion Mid-Barataria Sediment Diversion planned along Plaquemines Parish’s west bank, both diversions’ primary purpose would be flood control in addition to reconnecting nearby wetlands to the river’s sediment and nutrients. The Union Diversion would benefit the Maurepas Swamp area, cutting into the river’s east bank, while the Ama Diversion would send water through the west bank into the upper Barataria Basin.

“The results of this study are very promising,” said Tulane coastal and science engineering professor Ehab Meselhe, who is leading the ongoing study. “They show the opportunity to leverage these projects to benefit both Louisiana, through restoration and conservation of vital wetlands, and Mississippi, by limiting the flow of water through the Bonnet Carré Spillway and resulting increases in salinities across the Pontchartrain Basin and the Mississippi Coast.”

In 2020, Meselhe’s team found that operating the Ama and Union diversions together could have reduced the amount of water flowing through the Bonnet Carré by up to 60%. This study built on those results using a three-dimensional model capable of comparing how salinity changes during the 2019 flood might have differed had both diversions been operating.

Environmental Defense Fund scientist Devyani Kar said that had the diversions existed, the water would likely have remained twice as salty in a time when the water off Mississippi’s coast was nearly fresh. In the coming months, the researchers will use the model to see how the diversions would affect the concentration of nutrients flowing through the spillway, as well as temperature changes and other measures of water quality.

“We expect that when we’ll see more nutrients dispersed higher up in the marshes which need them and less nutrient load in the river,” Kar said. “The hypothesis is that it will reduce all the nutrients going into the Gulf of Mexico, which can create those harmful algal blooms.”

Coastal Protection and Restoration Authority Executive Director Bren Haase agreed.

“You're shunting that water through wetlands that can take it. They slow the amount of time it takes for that water to travel to the coast,” he said. “The water is cleaned essentially by the wetlands that it passes through, and so there's the benefits to the water and to the downstream areas that are receiving the impacts of that water.”

The wetlands also would receive nutrients they’ve been cut off from for nearly a century in some cases, Haase said.

Louisiana is a defendant in lawsuits filed by Mississippi cities, towns and business groups harmed by the spillway’s opening in 2019 alongside the Army Corps, and river management has been a key priority in Gov. John Bel Edwards’ second term. Currently, both diversions are far from becoming a reality, but the coastal authority has included the Union Diversion in its 2023 annual funding plan and is currently conducting a feasibility study.

Haase said the agency will be assessing possible locations for the diversions as well as various flows, working with the Tulane team that crafted the model. In addition to the diversions, the agency has also asked and pushed the Army Corps to conduct a comprehensive study of the lower Mississippi River Basin starting in Illinois to see where there might be opportunities to give the river more room.

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