NASA: Atchafalaya deltas are growing, not shrinking

Staff Report
MORGAN CITY — A NASA article published this month explores the growing deltas at the mouths of the Atchafalaya River and Wax Lake Outlet, which the article says are exceptions to widespread land loss across Louisiana’s coast.
Though land losses are widely distributed across the 200-mile wide Louisiana coastal plain, the Atchafalaya Bay stands as a notable exception, according to a NASA Earth Observatory article published in February.
In a swampy area south of Morgan City, new land is forming at the mouths of the Wax Lake Outlet and the Atchafalaya River, the article said. Wax Lake Outlet is an artificial channel that diverts some of the river’s flow into the bay about 10 miles west of where the main river empties, the article stated.
Port of Morgan City Executive Director Raymond “Mac” Wade said one of the biggest benefits of the growing deltas in the Atchafalaya Bay is that all the material dredged out of the Atchafalaya River goes for beneficial use. “We have disposal areas that the material’s pumped in, and a lot of the reason the area’s building up below Morgan City is because of the dredging,” Wade said. The dredge material has been used to build lots of barrier islands in the Atchafalaya Bay, which have lasted many years, Wade said.
The river sand dredged out of the river is what builds the islands, Wade said. “The fluff material we have in the channel, it’s a very, very, light, light, fine material. And it will not really build islands because it won’t stack it’s so fine,” Wade said.
The fluff “could be used if large dikes were built to pump the material into,” Wade said. “Building the dike area, that would cost more money to do all that,” Wade said. The U.S. Army Corps of Engineers uses the cheapest means available, he said.
The fluff consists of the light soil particles that wash down the river, Wade said. “As these light particles hit the saltwater, then they have a tendency to break apart and they settle into the (bar) channel,” Wade said. That “fluff” is what creates navigation issues for vessels, he said.
“When they’re pumping sand, all that sand builds barrier islands for us down there, which helps.” The building of those barrier islands is the reason the mouth of the Atchafalaya River is growing, Wade said.
“Could Terrebonne use this in their marsh restoration? The problem is it would cost too much to transfer all the way over there, 20 to 30 miles away,” Wade said. “It’s going to cost you way more than it’s worth to make an acre of land.”
Wade emphasized that the Atchafalaya delta is not growing by itself. “The corps has gone in there and dredged and used that spoil to make all these barrier islands down there,” Wade said.
St. Mary Levee District Executive Director Tim Matte said, “From St. Mary Parish’s perspective, the accretion that’s taking place out there is providing an additional buffer to the communities from storm surge and from the waters of the Gulf of Mexico so it’s a plus in that particular regard.”
The Terrebonne marshes, to the east of St. Mary, have degraded dramatically, Matte said.
“That’s providing less cover for us, less buffer, if you will, and making us more vulnerable to storms coming from that direction, whereas those that might have to cross over the Wax Lake Outlet or the Atchafalaya Delta, would have much less impact because you would dissipate some of the energy of the storm surge,” Matte said.
Overall, though, the delta plain of the Mississippi River is disappearing, the NASA article said. The lobe-shaped arc of coastal land from the Chandeleur Islands in eastern Louisiana to the Sabine River loses a football field’s worth of land every hour. Put another way, the delta has shrunk by nearly 2,000 square miles during the past 80 years, the release said. “That’s as if most of Delaware had sunk into the sea,” the article stated.
Sediment carried by the Atchafalaya River is building both deltas, the article said. Studies of the geologic history of the meandering Mississippi have shown that, if left to nature, most of the river’s water would eventually flow down the Atchafalaya, the article stated. But the Old River Control Structure, built in the 1960s by the U.S. Army Corps of Engineers, ensures that only 30 percent of the Mississippi flows into the Atchafalaya River, while the rest keeps moving toward Baton Rouge and New Orleans, the article said.
Even the Atchafalaya River’s flow has been sub-divided. In 1941, the corps opened the Wax Lake Outlet, a dredged channel designed to reduce the severity of floods in Morgan City, the article stated. About 40 percent of the Atchafalaya’s discharge gets channeled through the Wax Lake Outlet.
Even with the reduced flow, the Atchafalaya River carries enough sediment to build land, the article said.
While geologists first noticed mud deposits building up in Atchafalaya Bay in the 1950s, new land first rose above the water line in 1973 after a severe flood.
Since then, both deltas have grown considerably. According to one estimate by scientists from Louisiana State University, the Atchafalaya and Wax Lake Outlet deltas have combined to grow by one square mile per year, the article said.
The rate of land growth has varied considerably, mainly due to the timing of major floods and hurricanes, according to the NASA article.
Floods transport large volumes of extra sediment to Atchafalaya Bay, while hurricanes redistribute sediment within the bay and transport it offshore into deeper waters. Hurricanes also destroy coastal vegetation that would otherwise protect land from erosion, the article stated.
The two deltas added a combined 13 square miles of land between 1989 and 1995, for instance, but lost two square kilometers between 1999 and 2004, according to the LSU team. The land loss coincided with a period when tropical systems Allison, Isidore and Lili battered Atchafalaya Bay and there were no major floods to replenish sediments.
As is typical of deltas in southern Louisiana, the Atchafalaya and Wax Lake Outlet deltas have grown southward into the Gulf of Mexico, the article said. While the newer, outer lobes are periodically submerged by the sea and are still free of vegetation, various types of plants, including reeds and willows, colonize the higher elevations of the sandbars. Vegetation is critical to maintaining new land because roots stabilize sediment and prevent erosion, the article said.
The Atchafalaya delta has grown at a faster rate than its neighbor, about 0.6 square miles per year, versus 0.46 square miles per year for the Wax Lake delta, the article said.
The difference is due to regular dredging and channel widening on the lower Atchafalaya, which delivers extra sediment to its delta, the article said.
The emergence of new dredged islands is particularly noticeable between 1993 and 1994, the article said. Due to the lack of dredging, Wax Lake delta is more natural in character, with a more symmetric, lobate shape, the article said.
The key reason why the deltas in Atchafalaya Bay are growing while the rest of Louisiana’s coastline is retreating is that the Atchafalaya River brings sediment to the coast at a pace that allows it to settle into shallow water and to maintain marshes, the article said.
In contrast, an extensive series of levees keep the Lower Mississippi River’s water flowing in a narrow channel that whisks water and sediment past natural floodplains, the article stated. Instead of building new land along the mouth of the Mississippi, the controlled river sends jets of sediment-rich water directly into the relatively deep waters of the Gulf of Mexico and toward the edge of the continental shelf, according to the article.
With sea level rising rapidly due to natural geological processes, climate change, and human activities, predictions for the future of the Mississippi Delta are grim.
Even the land gains in Atchafalaya Bay will do little to offset the losses elsewhere, according to geologists. Most scientists expect the Mississippi Delta Plain to lose roughly 5,000 square kilometers of land over the next 50 years, according to the article.
“We are looking carefully at the Wax Lake and Atchafalaya deltas as models for building new land and preserving some of our coastal marshlands,” Harry Roberts, director of the Coastal Studies Institute at LSU, said in the article.
“If we start diverting significant portions of the water and sediment from the main channel of the Mississippi River into adjacent wetlands, lakes, and bays — as happens now in Atchafalaya Bay — we’ll be taking an important first step toward saving a significant part of Louisiana’s coastal plain.”