Louisiana's Coastal Marshes are Melting Away
November 03, 2010
Drowning the delta.
Five million ducks and a million geese come down to the Louisiana coast for the winter. They finish their molt into breeding plumage, choose mates, and, with a little luck, lay down some of the fat they'll need for spring migration and nesting. You could say that the foundation of next year's fall flight in the Mississippi Flyway rests on the bottomless black ooze of Louisiana's coastal marshes.
Which is one of the reasons waterfowl biologists and other conservationists are worried. The marshes are disappearing at the rate of more than 25 square miles a year. "We're on the verge of seeing an ecosystem collapse," says Phil Bowman, assistant secretary of the Office of Wildlife for the Louisiana Department of Wildlife and Fisheries. According to Bowman, Louisiana could lose 1,000 square miles of marsh in the next 50 years.
The Mississippi River has been adding land to Louisiana for millions of years. The result is more than 6,000 square miles of marshes from the eastern edge of the river's deltas to the low-lying Chenier Plains farther west. Forty percent of all the coastal marsh in the United States is here.
The highest ground in this wetland wilderness may be 10 feet above the water; most of it is a tangle of grass, sedge, and other plants rooted in the flooded muck left by the river. The Mississippi's normal flow is more than four million gallons per second. As a result, many of Louisiana's coastal marshes are freshwater systems, supporting a huge variety of plants and animals. Marshes closer to the sea turn brackish and then salty. These saltwater environments are harsher, but they still support a dense community of living things. The vegetation and invertebrates in and around these marshes are a smorgasbord for waterfowl. Huge rafts of lesser scaup feed on invertebrates in the shallow open water of the Gulf itself. The freshwater components of the marsh system support flocks of gadwall and teal along with significant numbers of wigeon, pintails, and mallards. This is the stronghold of the mottled duck--sixty percent of all the mottled ducks in North America depend on the marshes of the Louisiana coast.
Other game birds also use the marshes. Woodcock winter in much of Louisiana's wet country and are common on the coastal marshes. Losses of wet habitats in this region have probably played a role in the long-term decline in woodcock numbers. Rails, gallinules, and snipe flock to the coastal wetlands.
The list of nongame birds in the region contains more than 300 species, and many of them depend on the coastal marshes at some time of year. More than 180 species stop on the low ridges of the Chenier Plains in the fall to rebuild their stores of fat before continuing across the Gulf or around the coast into Mexico. Birds coming north across the Gulf in the spring will skip over the Chenier country if the weather is good but fall gratefully on it if they've been bucking a head wind across the ocean.
It's been estimated that the Louisiana coast provides 30 percent of the nation's commercial take of fish. Enjoy those Maryland blue crabs? Chances are good that they came out of Louisiana's coastal marshes. These wetlands are crucial nurseries for shrimp, crabs, and crayfish as well as redfish, seatrout, flounder, and several species of catfish along with popular sport fish like largemouth bass, crappie, bluegill, and redear sunfish. And the band of vegetation along the coast acts as a buffer, protecting the low-lying areas inland from the Gulf's nastier moods. The force and reach of a hurricane's storm surge are blunted by 50 miles of saltgrass, maidencane, and cattails. While New Orleans has moved mountains of dirt to protect itself from the ravages of high water on the Mississippi, it has taken the protection of the marshes to the south for granted. As the marshes shrink, disaster experts in the region are finally beginning to consider the consequences of another hurricane like Camille, and the predictions are sobering.
The thriving oil and gas industry along the coast is just as exposed. The marshes have protected major pipelines from the wave action of the Gulf. Without the marsh, the pipelines would be quickly scoured out of the muck and broken.
A variety of forces are eating away these wetlands. The first has probably always taken a toll--subsidence. When the river lays down its burden of sediment, there is lots of water in the mix. As the weight of overlying mud bears down, individual grains of sediment are pressed together, squeezing the water out. The mud settles, and the whole delta sinks a little.
For untold thousands of years, this sinking was more than balanced by the constant addition of new sediment carried down by the river, but in the last century, we've spent billions of dollars to change that. In the early 1900s, the Corps of Engineers began building dams on the upper river and its tributaries, partly to control flooding, partly to deepen the channel for commercial navigation. The dams are excellent sediment traps--they've reduced the amount of sediment arriving at the mouth of the Mississippi by more than 70 percent.
The Corps' traditional dual mission--flood control and improved navigation--have reshaped the Mississippi delta in other ways. Following the gigantic killer flood of 1927, the Corps redoubled its effort to contain the river with levees along the banks. When the river showed signs of shifting its flow into the lower Atchafalaya drainage in the 1950s, the Corps was there to build a huge diversion dam and lock, stopping the change and forcing the river to stay in its traditional channel. This gigantic project, combined with a system of levees downstream and constant dredging, has maintained the existing deep-water route to New Orleans and protected the city's status as a major port. The combination of these manipulations has worked a powerful change on the delta.
The river once shifted back and forth, dropping its sediment load on the continental shelf--now, it flows down the conduit the Corps has created, dropping its sediment off the edge of the shelf into the deep water of the Gulf. Without the constant addition of new sediment, there is nothing to compensate for the natural settling of the marsh bottom. As a result, millions of acres that once supported expanses of marsh vegetation are now open bays.
The channeling of the lower Mississippi has also reduced the amount of fresh water that spreads through the coastal marshes. Salt water seeps in to fill the loss and kills freshwater vegetation. The transition is often so rapid that the marsh bottom is washed away before saltwater plants can establish themselves. This leads to more wetland loss.
Early oil and gas development on the delta made the situation even worse. In the 1950s, 1960s, and 1970s, drillers dredged deep channels into the marsh so that they could move barges in as work platforms. The dredging destroyed thousands of acres of marsh directly; the channels allowed salt water to penetrate far into the wetland complexes, and the spoil banks interfered with the natural movement of fresh water in the marshes on either side. Spurred by public outcry, the companies have found ways to soften their impact, but the older drilling techn
iques are still taking a toll on wetlands.
About the same time the roughnecks were cutting their way into the marsh, some public-spirited individuals decided to import the South American nutria, a furbearer that has eaten its way through thousands of acres of wetlands and wooded swamps. Exotic plants like alligator weed and water hyacinth have also taken a toll, crowding out the native vegetation that largely defines the character of the wetland.
Along with many other conservation veterans, Phil Bowman has spent much of his professional career identifying the causes of wetland loss along the coast and trying to soften their impact. "We're learning from our mistakes," he says philosophically, "but we have to live with our mistakes."
There are efforts to stem the loss. The modern Corps of Engineers has helped build high-tech diversions to lift fresh water and sediment out of the river, over the levees, and into the surrounding marsh. While these projects are good for coastal wetlands as a whole, at least one created a new controversy when it changed the distribution of oysters and triggered a lawsuit from oystermen. Lower-tech approaches are also being developed. Ducks Unlimited has funded "duck-wing terraces" along the Chenier Plain to help protect marsh edge from the waves of the Gulf. The Louisiana Department of Wildlife and Fisheries and a number of partners are experimenting with breaks in the natural levees along the Mississippi. These breaks, called "crevasses," allow fresh water and sediment to flow out into the marsh. As the water slows, the sediment drops to the bottom, creating a micro-delta that quickly develops its own marsh vegetation.
The problem with these solutions is a matter of scale. They are creating hundreds of acres of new marsh while tens of thousands of acres are being lost. It's been estimated that an adequate program for protecting Louisiana's coastal wetlands will cost $14 billion. A coalition of conservation groups and government agencies called America's Wetland is trying to convince Congress to throw its weight behind the effort.
The loss of Louisiana's coastal marshes is a national crisis. Our communal attempt to control the Mississippi, Missouri, Ohio, and Tennessee rivers started the damage; our insatiable appetite for oil and gas has made it worse. If the Louisiana wetlands disappear, the effect will be felt far beyond Houma and Venice, Lake Charles and New Orleans. The wealth of these marshes--redfish and shrimp, gadwalls and pintails, whitefronts and snow geese--finds its way to the farthest corners of America. The delta touches us all. And if we're shortsighted enough to let it go, we'll feel the loss as well.