MRGO Closure Has a Greater Impact on Coastal Restoration and Recovery Than Ever Imagined

10.04.2019 | In Coastal Restoration
By John Lopez, Director, Coastal Sustainability Program, Lake Pontchartrain Basin Foundation

After decades of decline, the Pontchartrain Basin is ready for restoration as a result of the closure of the Mississippi River Gulf Outlet (MRGO). The closures, a navigational closure and a surge barrier, have managed to bring historical salinity gradients back to over a million acres of coastal habitat. This means that the basin will be able to support a variety of species, ranging from cypress trees to oysters, while work is done to restore the vital storm surge buffer that forested wetlands and marshes provide nearby communities, including the Lower 9th Ward and St. Bernard Parish.

On Louisiana’s coast, good news is not always easy to come by; however, since the closure of MRGO, salinities in the basin have returned to a range that can support a full spectrum of plant and wildlife species, from freshwater to saltwater, while reducing the devastation of storm surge with restored wetlands. While there is still a long road to restoration in the vast coastal area impacted by the MRGO, the closures have already had significantly positive impacts across the basin. 

How closing MRGO is helping achieve the remarkable

The two closures across the MRGO have reduced the exchange of both freshwater and saltwater. So, what is the net effect to our estuary and salinity gradient? With the MRGO closures, LPBF assessed suitability for both cypress habitat in the northern end of the basin and for oysters in the southern end of the basin as a result of the new, post-closure salinity and hydrology composition. The results are remarkable: 

Freshwater swamps are returning: By lowering salinity, the MRGO closures have made approximately 150,000 acres of coastal land suitable to replant trees and restore swamps.  Fortunately, these areas tend to be where swamp restoration could reduce risk of flooding from storm surges. For example, the land bridge separating Lakes Pontchartrain and Maurepas is a “critical landscape feature” identified by the Corps of Engineers because it reduces storm surge. Since the closure of the MRGO, the habitat has improved dramatically. LPBF and its many partners are aggressively doing swamp restoration here to strengthen this important coastal buffer. We have planted 36,000 trees with an 80% survival rate! Last year, we expanded our restoration efforts into the LaBranche Wetlands in front of the St. Charles Parish hurricane levee. (Volunteer to help us!)

Bottom-line is that the closure of the MRGO helped revive the fresh-end of Pontchartrain estuary, but what about the salty end?

Oysters return to historical habitat: LPBF has conducted a series of studies and reports identifying areas suitable for oysters since 2013. In addition, we have carefully researched where oysters were located before the MRGO was dug.  It’s a near perfect match. Since the closure in 2009, the areas that are now prime oyster habitat and where oystermen now work are nearly identical to where oysters were located before the MRGO was opened. The two maps of the Biloxi Marsh below illustrate that between 2013-2018 oyster reefs were almost perfectly reestablished in their ancient oyster beds.  We expect this area to produce commercial oysters for many years to come. In fact, LPBF and the Louisiana. Department of Wildlife and Fisheries are in the process of building four reefs in or around the Biloxi marsh to facilitate oyster growth here.

Remembering MRGO’s catastrophic legacy: While these developments are hopeful, it’s important to remember the widespread damage MRGO caused to our region. It’s been nearly 15 years since Hurricane Katrina funneled storm surge into communities across the Lower 9th Ward and St. Bernard Parish, causing unimaginable death and devastation. In the decades prior, these communities had their wetland buffer decimated by saltwater intrusion from MRGO. After Katrina, the communities are forever changed.

In addition to the human impact, MRGO massively altered the water flow of our estuary. While the channel was open, it dominated the Pontchartrain Basin, causing large-scale saltwater intrusion into the upper part of the estuary that was formerly fresh.  The collapse of the salinity distribution was so drastic that there is an area in the Central Wetlands where salt-loving oysters could be found growing on freshwater-loving cypress knees and cypress stumps!  This represents a near complete ecologic collapse and alteration to our estuary. It’s like polar bears in a rainforest. It ain’t right.

Oysters on Cypress knees while the MRGO was open

During this time of estuary collapse, oystermen made a living just a few miles away in Lake Borgne, where saltwater intrusion from MRGO had allowed oysters to exist where they wouldn’t have before the channel was dug.

Because of the devastating effects of the MRGO during Hurricane Katrina, two dam-like closures were built across the MRGO channel in 2009. The rock dam (5-foot-high), a navigational closure near Hopedale, reconnected the old Bayou la Loutre Ridge, which is a natural partition to the water within this area of the estuary.  The rock dam generally reestablished this natural ridge function as a barrier to tidal flow. The second closure is the Lake Borgne Surge Barrier, which is a solid, 28-foot-high concrete wall.  The surge barrier is part of the flood protection system around the Greater New Orleans region and is the most significant addition to flood protection after Hurricane Katrina. The system provides critical protection to St. Bernard, New Orleans, Jefferson and St. Charles Parishes. The surge barrier across the MRGO is permanently closed, but nearby two channels have gates that allow for boat traffic and normal tidal flow. 

Looking ahead

Despite the recent return to a balanced Pontchartrain Basin, 2019 was an outlier because of the unprecedented flood on the Mississippi and Pearl Rivers and two openings of the Bonnet Carre Spillway. The combined influences pushed freshwater further east than ever before, almost to the Chandeleur Islands. The floods pushed the Biloxi Marsh to lower salinity levels than normally seen with the MRGO closures, and reflects an extreme inundation by exceptional events. Despite this massive, record-breaking spring and summer flood, we are seeing estuary conditions return to normal in the basin this fall. Yes, we can have too much freshwater and we can have too much saltwater, but the estuary must have a normal annual swing of fresher to saltier conditions.

The closure of the MRGO has accomplished an amazing feat of restoring much of the hydrology of our estuary. Today, conditions in the Pontchartrain Basin are closer to historical records than they’ve been since the 1960’s and that is a hopeful sign for Louisiana’s coast. Closing MRGO is helping to restore salinities and hydrology to allow for healthier, more diverse habitat. Now, we are more ready than ever for the marsh, swamp, ridge, and oyster reef restoration of the basin’s wetlands that will provide storm protection to over a million people in the region! Learn how you can support this work here.