The Pearl River’s flow of fresh water moderates salinities in tidal waters that nourish the Hancock County Marshes in Mississippi and the Biloxi Marshes of Louisiana. The Biloxi Marsh is that big chunk of St. Bernard Parish that sticks up to the north, lying due south of Waveland, Mississippi. Bayou Biloxi runs through part of this critically important marsh. It is this finger of Southeast Louisiana that is the main buffer for storm surges from the east and southeast. It is a line of defense that benefits both states.The Mississippi River created the St. Bernard Delta, its easternmost lobe, between 2000 and 4000 years ago. Manmade River levees block Mississippi River sediment from this part of St. Bernard Parish. It was the yearly flooding of the big river that built this area and much of southeast Louisiana. The Corps of Engineers built diversion projects at Violet and Caernarvon to move freshwater from the Mississippi River to some marshes and oyster reefs in the region. Despite these engineering efforts, the Biloxi Marshes are shrinking from erosion and subsidence.New marsh won’t grow without new sediment, and the distant Biloxi Marshes won’t receive much. We must care for what remains. The fresh water supply that moderates salinities in these marshes needs to remain stable. The main distributary channel for moving fresh water into the Biloxi Marsh complex is Bayou La Loutre in St. Bernard Parish. It was leveed off from the Big River more than a century ago. It was cut in two by the Mississippi River Gulf Outlet that cut through St. Bernard Parish in the 1960s. The fresh water streams that used to branch from Bayou La Loutre are not delivering much fresh water into the heart of these marshes. As it stands, rain, fragmented streams, and the Pearl River are the only sources of water for these parched marshes.The Pearl River has the fourth largest fresh water discharge to the Gulf east of the mouth of the Mississippi River. The Pearl has a larger fresh water flow into the coastal estuaries than the combined rivers of the Lake Pontchartrain basin. This is because the Pearl River has a larger watershed than the Pontchartrain basin streams.With the help of Mississippi’s Barrier Islands, the Pearl River moderates salinities in the Western Mississippi Sound and Lake Borgne. The islands: Horn, Petit Bois, Ship and Cat Island act as a curtain to block full strength sea water from moving into the Mississippi Sound and the Rigolets. Ongoing Corps projects to repair and rebuild these islands will help keep salinities moderate in the Sound north of them.The Biloxi Marshes in Louisiana need to remain as healthy as possible. Subsidence, a lack of new sediment, and the attack by wave action and storms are taking their toll. Periodic storms wreck marshes. Hurricane Katrina’s winds and surge plowed across the Biloxi Marshes. Both states have armored the marshes with rock or living shorelines in the decade since Katrina. The Biloxi Marsh will only grow more important as a storm buffer for the western Mississippi coast and the eastern approaches to New Orleans.Oysters are economically important in this region and grow in and around the Biloxi Marshes in both states. High salinity water brings marine predators like the oyster drill snail to oyster reefs. The longer the salinities remain high, the more damage oyster drills do. Full strength sea water is 33 parts per thousand (ppt). Oysters can live in the 10-30 ppt range, but 15-18 ppt is optimal. Oyster production has declined in Mississippi since Katrina. The oyster reefs near the mouth of the Pearl River need restoration after dying off in waters with low oxygen during summer and fall this year. Salinities in in the Western Mississippi Sound can become quite high at the end of every summer, during the annual low flow period for coastal rivers. This year, rain was scarce between August and November, and salinities there rose sharply. Whether it’s the lack of rainfall or upstream engineering, reductions to fresh water flow can harm oysters.The State of Mississippi’s decision makers may not seek to know about the geology of the St. Bernard Delta, or the present hydrological challenges facing the Biloxi Marshes, but Mississippi should know enough to avoid harming its oyster and seafood industry by choking off fresh water flows to the coast. In fact, there are some that do know and are acting. The 2015 final report of the Mississippi Governor’s Oyster Council recommends us to “educate decision-makers on impacts of major freshwater-depleting projects” . This includes threats from new dams on coastal rivers that could harm oysters and other coastal resources. Dams create lakes that speed up evaporation, impede flows and disrupt sediment and nutrients moving down river systems.Decision makers can certainly use some education. The state of Mississippi exhibits schizophrenia about its coastal resources. It gives a green light to water projects that can work against one another. The one hand has granted $1 million from the Mississippi Development Authority to the Rankin Hinds Pearl River Flood Control and Drainage District to study a lake project for providing flood control on the Pearl River in Jackson. The other hand, the Mississippi Department of Environmental Quality, has supported $50 million of marsh shoreline protection, and restoration of marshes and oysters at Heron Bay, a mile from the mouth of the Pearl River in Hancock County, Mississippi. Two other BP restoration projects are ongoing in Louisiana in the Biloxi Marshes and in the eastern New Orleans marshes. The success of much of this restoration depends on moderating salinities, and for this to happen, the seasonal fresh water flows from the Pearl need to be dependable.So… tinkering with the Pearl River’s fresh water discharge by building a new lake upstream of the coast is not smart if the state cares about coastal resources and wetlands. The Pearl’s fresh water discharge is very important, given the threats to marshes near the river’s mouth. The stability of the Biloxi Marshes of St. Bernard Parish and the benefits of the undiminished fresh water discharge from the Pearl River accrue to both states and to New Orleans which needs storm surge reduction. Because they were built by river deposition, having more, not less, fresh water flowing into them is better for sustaining the Biloxi Marshes. This goes for the Hancock County marshes in Mississippi as well.Look east at what Atlanta’s water use is doing to the Apalachicola, Chattahoochee, and Flint rivers systems and review the damage to oyster reefs there from high salinities in the coastal estuaries of Apalachicola Bay. Florida, Georgia and Alabama have been locked in legal battles over water for a decade or more. A shrinking oyster industry bears the most weight. Mississippi should take a lesson from this nearby example and protect the Pearl River’s flow that sustains coastal estuaries and marshes. If Mississippi won’t do this, Louisiana must weigh in.Louisiana and Mississippi both have a dog in this fight as the Biloxi Marshes protect both states’ coastal areas from storm surges. Coastal land loss from subsidence and sea level rise is a significant problem in Louisiana, and to a slightly lesser degree along the Mississippi coast. Caution should prevail, and both states should be very conservative when considering upstream projects that can deplete fresh water. Andrew Whitehurst is GRN’s water program director and covers Mississippi wetland and water issues. Scott Eustis’ editing of coastal wetland information is appreciated.