Mississippi Sound oysters, near the Alabama border, are at risk by the diversion.

Freshwater from Louisiana’s proposed $800 million Mid-Breton Sediment Diversion could pose a significant threat to oysters and other fisheries in Mississippi Sound, according to a new study commissioned by the Mississippi Department of Marine Resources. When the diversion’s water is added to an average of Mississippi River, Lake Pontchartrain and other Mississippi coast rivers and bayous obtained from an 11-year record of water flows, salinity levels in portions of the western Mississippi Sound near Bay St. Louis where oysters are grown drops to unsafe levels for as much as 50 days or more, said Jerry Wiggert, lead author of the study, and associate director of the University of Southern Mississippi School of Ocean Science and Engineering. Oysters are generally able to tolerate salinity levels of between 5 and 15 parts per thousand. However, the peer-reviewed study conducted by Wiggert and other USM researchers was based on Louisiana’s original plan for the diversion that called for a maximum flow rate of 75,000 cubic feet per second. In 2021, the state announced it was reducing the diversion’s maximum flow to 50,000 cfs.

This map of the Mississippi and Chandeleur sounds and Lake Borgne on the Louisiana and Mississippi coasts shows the model results for an 11-year average of Mississippi River flows when the proposed Mid-Breton Sediment Diversion operates at a maximum 75,000 cubic feet per second, and the Bonnet Carre Spillway is not opened. The bar at right is the difference in days with salinity levels below 5 parts per thousand, the level where oysters are harmed.
(University of Southern Mississippi) 

There are still areas to look at but they are certain water will be changed.

Wiggert said his team has not looked at that lower maximum flow rate, but expects it could result in as much as 21 days of salinity levels below 4 parts per thousand in the western sound, which would still be a threat to oysters. The diversion would be built just south of Wills Point on the east bank of the Mississippi River in Plaquemines Parish in hopes it will add 31 square miles of new land in Breton Sound by the end of 50 years, with a key goal of reducing the effects of storm surge on the area’s east bank levee systems, in addition to adding fish and wildlife habitat. It is one of two large-scale diversions currently moving toward construction as the state seeks to slow coastal land loss. The other, the Mid-Barataria Sediment Diversion on the opposite side of the river at a cost of more than $2 billion, is expected to begin construction later this year. The Mid-Breton diversion’s gates would be open when the Mississippi River’s flow at Belle Chasse reaches 450,000 cfs, and would reach its maximum flow when the river exceeds 1 million cfs. It would be expected to operate for as much as nine months during the year, though rarely at its maximum flow. Up to 5,000 cfs would flow into the diversion channel when the gates are officially closed.

An illustration released Wednesday, Nov. 17, 2021, shows how the proposed Mid-Breton Sediment Diversion would funnel a maximum 50,000 cubic feet per second of water and sediment from the Mississippi River into Breton Sound. Officials said the conveyance channel might be shortened to reduce the project’s cost.

The CPRA will look at the results but seemed to have been blindsided.

The Louisiana Coastal Protection and Restoration Authority, which would build the diversion, said that it planned to look into the USM study, but expressed concern it was not contacted to help design the model used by the researchers or their investigation plans. “There are key pieces of technical information we could have provided that would have improved the rigor of the modeling tool and the resulting analysis, and that would have fundamentally altered the conclusions put forward,” the statement said. That included the lower flow level, which the state said it had shared with Mississippi officials in meetings on the diversion. “From the modeling that we’ve conducted in support of the project’s environmental impact statement, salinity impacts are reduced substantially with that capacity reduction,” the statement said. “The USM study also focused only on modeling current conditions, and didn’t consider the 50-year analysis period CPRA modeled for the project, which is meant to combat future issues like saltwater intrusion and emergent wetland loss from rising sea levels.”

These are potential issues that are to be addressed in the environmental impact study for the federal permits required for the proposed Mid-Breton Sediment Diversion.
Army Corps of Engineers

Sea rise may help offset this concern.

Sea levels along the Mississippi coast are expected to be at least a foot higher at the end of 50 years, which would likely increase salinity of in the sound. The report’s results have prompted reactions from Mississippi Sound oyster growers and other fishers, and from Mississippi state officials. “This proposed diversion is an ecological death sentence,” said Tac Carrere, executive director of the Mississippi Sound Conservancy, which represents a variety of fishery and other users of the sound. “Having witnessed first-hand physically the destruction to the Mississippi Sound that resulted from the Bonnet Carre Spillway opening, it’s completely obvious how the proposed Breton Sound diversion if implemented as planned will be catastrophic.” Carrere said Louisiana should refocus its funds to projects that build new wetlands with dredged sediment. Mississippi Secretary of State Michael Watson said the study shows the need for the Corps to include impacts to the Mississippi Sound in its environmental analysis of the diversion and for representatives from his state to be involved in determining what harm it may cause. But Watson also said he recognized Louisiana’s need to consider building it. “Because of the delicate balance existing in our shared ecosystem with our neighbor, we can’t ignore the existential land-loss problem Louisiana is facing in the Mid-Breton Sound and Biloxi Marsh,” he said in a statement. “As healthy marsh lands continue to disappear across what is left of the crow foot delta, we must determine what the long-term impacts to the (sound) will be when they are gone. While we have evidence of the negative impacts (the diversion) could have on the (sound) under the studied operational parameters, a course of no action on Louisiana’s part could also have disastrous effects on the Mississippi Gulf Coast.”

This map shows the potential growth in land height after 50 years resulting from operation of the proposed Mid-Breton Sediment Diversion. This is based on early modeling results of sediment deposition and is not final.
Coastal Protection and Restoration Authority

Research continues but the use of Louisiana republicans talking points by Mississippi makes we wonder about this study and its timing.

Wiggert said his team already is discussing additional model runs using the 50,000 cfs maximum now proposed by Louisiana. The USM study also looked at what effect operating the diversion at maximum capacity would have had in 2019, a year with near-record flow down the Mississippi River that resulted in the Bonnet Carre Spillway being opened for a record 123 days. That year, the maximum flow through Bonnet Carre was 207,000 cfs, more than four times the maximum flow of the diversion. The eventual flow of that water into Mississippi Sound added to high flows from rivers and bayous along the Mississippi coast, including the Pearl and Pascagoula rivers, destroyed the sound’s oysters and caused harmful algae blooms that killed other fish and disrupted tourism to the tune of about $200 million.

The study concluded many things.

The study concluded that operating the diversion that year would have had little effect on salinity in the sound, as the water flow out of Lake Pontchartrain and rivers along the coast would have kept its waters from moving north, and their freshwater was more than enough to kill the oysters. The spillway has been opened six times in the last 15 years, compared to an average of only once every 10 years between its opening in 1932 and 2008. The repeated openings triggered lawsuits by Mississippi and the state’s local governments, including one that ended with the Corps being ordered to consult with the National Marine Fisheries Services before future openings. But in 2007, Mississippi officials were so concerned about salinity levels in the sound threatening oysters and other fisheries that then-U.S. Sens. Trent Lott, R-Miss, and Mary Landrieu, D-La., inserted language in legislation authorizing construction of a freshwater diversion near Violet. That diversion was never built. The state has applied to the Corps for permits for the Mid-Breton diversion, and studies by the Corps, including a comprehensive environmental impact statement, are barely underway. A Corps spokesperson said the state has informed it of the plan to move to 50,000 cfs, but has not yet amended its permit requests.

At first glance how could a diversion near the Mississippi impact oysters near Mobile. But I guess water spreads.

More oysters at risk