Louisiana’s rare growing coast, anchored by black willows and bald cypress trees

The Lens
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Satellite imagery shows how the Wax Lake and Atchafalaya River deltas have grown between 1984 (left) and 2017 (right). Photos by NASA Earth Observatory

It looks so natural. But the shape of the lower Mississippi River – the way it curves, winding like a snake – was carefully engineered. Its design provides water to industry between Baton Rouge and New Orleans and accommodates ship navigation leading out into the Gulf of Mexico through Plaquemines Parish.

Over the course of 7,000 years, as the Mississippi River built much of Louisiana, its shape shifted. The river meandered westward and then back to the east, delivering sediment that created six modern sub-deltas as the river continuously overtopped its landbanks, chasing the most direct route to the Gulf.

Around 1860, the mighty Mississippi began a slow careen back west, toward the Atchafalaya River. The U.S. Army Corps of Engineers stepped in, building levees, dams and locks that basically froze the moving river in time.

Without that intervention, the Atchafalaya would have continued to draw off more and more water from the Mississippi, until it became the master stream. The Atchafalaya’s dominance would have economically diminished everything that relied on the Mississippi, including Baton Rouge, New Orleans and the industries between the two.

“For nature to take its course was simply unthinkable,” wrote environmentalist John McPhee in 1987 . “Nature, in this place, had become an enemy of the state.”

In 1963, as the Army Corps continued its century-long effort to keep the Mississippi on its charted course, the agency completed Old River Control , a dam and lock system that controls how much of the river’s flow is captured by the Atchafalaya.

The system controls the volume of water that flows from the Mississippi and Red Rivers, based on how the river flow was divided in the early 1960s – with 70% continuing to New Orleans and 30% flowing into the Atchafalaya.

Twenty years earlier, in 1942, to prevent the Atchafalaya from flooding Morgan City, the Army Corps had divided that river’s flow, sending a portion of it through a newly dredged channel called the Wax Lake Outlet , which runs about 15 miles from its junction with the Atchafalaya to the Gulf of Mexico.

Then came the Mississippi flood of 1973, which nearly caused the collapse of Old River Control. The massive influx of water forced both the Bonnet Carré and Morganza Spillways to remain open for two months.

Those floodwaters also carried a surge of sediment to the subsiding coast.

That sediment formed land at the coast, in a state whose coastline is infamously disappearing . For 50 years, the juncture where the Wax Lake Outlet and Atchafalaya River meet the Gulf of Mexico has been creating new land. Sediment had been building underwater for a few decades. But as floodwaters receded in 1973, new land was first seen above the water’s surface.

A few years later, in a key part of that process, black willow trees showed up, to keep that land intact.

Today, satellite images show two individual lobes growing into the blue waters of the Gulf, in a subdelta where the mouth of the Mississippi drained thousands of years ago.

While Old River Control was not built as a sediment diversion project, it now provides scientists with a real-life model to study early delta formation. The Wax Lake and Atchafalaya deltas offer a glimpse of what could be possible with the construction of the Mid-Barataria Sediment Diversion in Plaquemines Parish.

Not everyone agrees with the newest engineered intervention. The parish is suing the state to halt work on the Mid-Barataria diversion due to the increased flood risk it poses to coastal communities. Earlier this month, Plaquemines Parish oyster companies delivered a notice of intent to sue alleging that the diversion violates the Endangered Species Act and will likely harm several listed species.

New Land and Life on the Atchafalaya and Wax Lake Outlet

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The Black Willow is a native North American tree that has historically been used for medicinal purposes, such as pain relief. Its bark is a source of salicin, which was isolated from European willows in the 1820s and used to create a synthetic form now sold as aspirin. Photo by Hannele Lahti / Smithsonian Gardens

Often at low tide, the newest land can be seen just above the water’s surface. At first, the clay islands act as a stopover for different species of birds. At this stage, the formations can bear the weight of a human, but the clay will sink and a shoe may be lost in the muck.

In new deltas, researchers can track the stages of land formation, through plants.

Because the Mississippi River and its deltaic marshes are so dynamic, the plants that live here have even evolved to rely on its changing cycles. Cypress trees, for instance, require a dry period in order to germinate. If the river is flooded for too long, the seeds will not be able to grow into mature forests.

Within swampland, land building starts smaller, with floating marsh plants, found at the water’s surface as sediment starts to build up underwater. The tightly entangled plants and their roots are mixed with peat but not anchored to the soil beneath, making it undulate when moved by tides or winds, becoming, as the Cajuns call it, la prairie tremblante , or the trembling prairie.

Floating marshes destined to become land will thicken. Larger plants grow on the flotant mat, which begins to slow down water flow and trap sediment. The land begins to bind together with the help of other early plants, grasses such as cordgrass and sea oats as well as baccharis shrub and swamp mallow flowers. They anchor roots in the soil and contribute to land building by further trapping sediment from the water column.

About two years after the island begins to form, black willows begin popping up along the margin of the marsh, where the elevation of sediment is higher. In these new environments, black willows are the first trees to take hold. They can grow quickly and their woody roots trap more sediment than grasses and shrubs, reducing land loss at a much greater rate than leafy marsh plants alone.

Black willow, a native North American tree, is still common enough in coastal Louisiana that it spreads naturally. Other native plants, such as cypress and tupelo trees, are less likely to respawn naturally because they have been erased from much of Louisiana’s coast through saltwater intrusion and widespread logging.

Bald cypress roots had long been a staple of Louisiana swamps, where cypress trees, like other coastal trees, dig deep down into the soil and make the area more resilient to storms. To help them return to the state’s swamps, the Coalition to Restore Coastal Louisiana and the Atchafalaya Wildlife Management Area planted 500 bald cypress trees late last year.

Marsh trees are also known for helping to absorb floodwaters and break high hurricane winds. Because of that, establishing wooded forests in swamp habitat between the Gulf and coastal communities would increase storm protection during hurricane seasons, said Gardner Goodall, the Coalition’s native plants program coordinator.

Last year, Louisiana Gov. John Bel Edwards nominated a portion of the Atchafalaya Coastal Basin to become the state’s first National Estuarine Research Reserve , which would support further scientific research of the two actively growing deltas.

The new NERR designation makes perfect sense to Alisha Renfro, senior scientist at the National Wildlife Federation, who has taken boat rides through the Wax Lake Outlet. “You really see and feel what a thriving marsh feels like, versus so much of our coastline that is dying,” Renfro said. Beyond its beauty, the Atchafalaya Basin is also environmentally significant, she said. “It’s really the only place in Louisiana where we are consistently gaining land instead of losing land.”

Swamps: helping the environment by holding carbon dioxide

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Andre Rovai, a research ecologist with the U.S. Army Corps of Engineers’ Research and Development Center, readies equipment on Mike Island, a part of the Wax Lake Delta that emerged in 1985 on the coast of St. Mary Parish. Black willow trees stretch out across the island in the background. Photo by Mizani Ball / The Lens
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Rovai injects liquid nitrogen into the ground to freeze a soil sample used to measure how much land has accumulated over the length of the four-year study. Photo by Mizani Ball / The Lens

Every six months, a team from Louisiana State University’s College of the Coast and Environment spreads a chalky substance known as feldspar in a marked area at the center of Mike Island, an island in the Wax Lake Delta that first emerged in 1985. The island is now thickly covered with swamp plants and black willows.

As more soil accumulates, the feldspar will serve as a marker – like a layer of peanuts in an ice-cream parfait – within the team’s vertical samples of soil, to observe how much sediment has settled on top of the chalky layer since the last measurement.

To take these samples, Andre Rovai injects liquid nitrogen into the ground to freeze a column of soil. These “popsicle sticks” can then be removed from the ground without being compressed. This ensures that the measurements remain more accurate.

For four years, Rovai worked in the Wax Lake Delta with LSU’s team. He’s now continuing that work as a research ecologist with the U.S. Army Corps of Engineers’ Research and Development Center in the wetlands branch of their environmental lab. So he knows that growth from year to year can be uneven. “Some years you get a lot of sediment loading from the river,” Rovai said. “Some years you get more storm activity that can erode deltaic floodplains that haven’t completely stabilized yet.”

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A “popsicle stick” created with liquid nitrogen and feldspar to measure soil accumulation in the last six months. Over the last few years, a period of drought and low river flow, Mike Island has accrued about two centimeters of soil. But in 2019, during a period of high river flow, 12 centimeters built up in one year. Photo by Mizani Ball / The Lens

As he works, Rovai also keeps an eye on the health of the trees around him, because, like new land, they can help in Louisiana’s struggle against climate change.

Beyond serving as a soil-binder for land-building, trees help Louisiana’s coastal marshes store a vast amount of carbon dioxide.

Leafy plants will decompose and release their stored carbon within a year. But the woody roots of trees hold onto carbon dioxide more effectively than other marsh plants. After trees die, carbon remains in tree trunks, which take a long time to decompose and can become covered in sediment, trapping the carbon in the soil for centuries in the form of peat.

The coastal marshes also absorb excess nutrients that end up in the Mississippi River as runoff from fertilizer used for farming in the Midwest. In places like the Wax Lake Delta, the additional nutrients can help spawn rapid growth of vegetation on newly formed land masses.

The nutrients, mostly nitrogen and phosphorus, constantly flow downriver and cannot be soaked up entirely because Louisiana’s coast has lost so much of its wetlands. The nutrient pollution often ends up plummeting into the Gulf of Mexico where it creates low oxygen areas that decimate sea life, known as the “ Dead Zone .”

The excess nutrients are not always a boon, however. Trees produce large, intricate root systems when their roots have to extend to absorb nutrients in habitats where nutrients are sparse.

When nutrient levels are high, the woody trees will dedicate more of their growth above ground, Rovai said. The resulting, smaller root systems can be dangerous for Louisiana’s coastal marshes, as they cannot hold the soil together as tightly and may not stand up as well to high winds.

A “living laboratory”

Because of their longevity – the Atchafalaya and Wax Lake deltas have now been growing for 50 years – Simone Maloz, campaign director for the Restore the Mississippi River Delta coalition, sees this thriving new land as a “living laboratory” for the rest of the state’s coast.

The state’s Coastal Master Plan , which looks ahead at the next 50 years for Louisiana, incorporated lessons learned from studying the early delta-building in the Wax Lake Outlet, which helped to form a major component of the plan, the Mid-Barataria Sediment Diversion in Plaquemines Parish.

Other lessons can be learned from the Mississippi itself, which replicated the Wax Lake Outlet model on its own, creating land naturally on a part of Plaquemines’ otherwise-disappearing coast.

In this case, the Army Corps was a mere witness to the process, as the Mississippi carved out a new crevasse – a breach in the river’s bank – to carry its water to the Gulf. Over the past six years, the small canal on the east bank of the Mississippi River has widened into a major canal, depositing sediment along the bank of the channel, known as Neptune Pass .

The Army Corps took notice of the crevasse in 2021 , when it began to divert so much water that it altered the Mississippi’s current. Now, the short but mighty channel, approximately the size of the Wax Lake Outlet, has a flow five times that of New York’s Hudson River. And where it meets the Gulf, about 70 river miles south of New Orleans, new land is creeping into Quarantine Bay.

The powerful flow of Neptune Pass has brought fresh mud, clay, sand and silt back to the saltwater bay. Vegetation and waterfowl have returned to the area by the fishing community of Buras, which lies close to Bay Denesse and was open water only a few years ago.

Robert Thomas studies nature all day, as director of the Center for Environmental Communication at Loyola University New Orleans. But he still marvels at the dynamic system created by the Mississippi and its tributaries. In the 1970s, when he started his career in coastal ecology, there was no delta whatsoever at the mouth of the Atchafalaya, he said. Since then, he has watched the system at work.

“It’s pretty amazing,” Thomas said. “To go down to the Wax Lake Outlet and see a massive delta that’s grown in my lifetime.”

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