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Sunday, June 16, 2019

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While Louisiana land loss continues, new lands appear in Atchafalaya Basin

While Louisiana land loss continues, new lands appear in Atchafalaya Basin

While most of the delta plain of the Mississippi River Delta is losing ground, new land is forming in Atchafalaya Basin at the mouths of the Wax Lake Outlet and the Atchafalaya River according to recent satellite images and a report released by the National Aeronautics and Space Administration (NASA).

The main delta plain of the Mississippi River continues to disappear. A 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 over the past 80 years, equivalent to most of Delaware sinking into the sea.

Geologists first noticed mud deposits building up in Atchafalaya Bay in the 1950s, but new land first rose above the water line in 1973 after a severe flood. Since then, both the Wax Lake Outlet and the Atchafalaya River deltas have grown considerably. According to an estimate by scientists from Louisiana State University (LSU), the Atchafalaya and Wax Lake Outlet deltas have combined to grow by one square mile per year.

Though land losses are widely distributed across the 200-mile wide coastal plain of the state, Atchafalaya Bay stands as a notable exception.

The Atchafalaya is a distributary of the Mississippi River that connects to the “Big Muddy” in south central Louisiana near Simmesport. Wax Lake Outlet, an artificial channel designed to reduce the severity of floods in Morgan City, delivers about 40 percent of the Atchafalaya’s water into the bay about ten miles west of where the main river empties.

A series of false-color satellite images chronicles the growth of the two deltas between 1984 and 2014.

Instruments on Landsat satellites acquired all images. A combination of shortwave infrared, near infrared, and green light was used to accentuate differences between land and water. Water appears dark blue; vegetation is green; bare ground is pink. All of the images were acquired in autumn, when river discharge tends to be low.

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. 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 of the river keeps moving toward Baton Rouge and New Orleans.

The deltas’ rate of growth has varied considerably, mainly due to the timing of major floods and hurricanes.

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 Atchafalaya Basinhas grown at a faster rate than its neighbor—about 0.6 square miles per year, versus .46 square miles per year for the Wax Lake delta. The difference is due to regular dredging and channel widening on the lower Atchafalaya, which delivers extra sediment to its delta. Due to the lack of dredging, Wax Lake delta is more natural in character, with a more symmetric, lobately shape.

LSU Sea Grant has been deeply involved with studies on the restoration of river deltas which involves diverting sediment and water from major channels into adjoining drowned areas, where the sediment can build new land and provide a platform for regenerating wetland ecosystems.

Except for local engineered structures at the points of diversion, restoration mainly relies on natural delta-building processes. Present understanding of such processes is needed to provide a basis for determining the feasibility of restoration projects through quantitative estimates of land-building rates and sustainable wetland area under different scenarios of sediment supply, subsidence, and sea-level rise.

“Hurricanes damage coastal vegetation, but most healthy marshes recover quite nicely,” said Robert Twilley, Director of the LSU Sea Grant program. “Our research team has been working in Wax Lake for the last eight years and have seen two hurricanes strike the new delta. In both instances the vegetation recovered in several months following the storm. This new marsh is very solid marsh that handles hurricanes very well.”

The world’s deltas are, in both literal and figurative senses, the thin end of the wedge in terms of coastal response to changes in sea level. Their typically low surface gradients make deltas potentially vulnerable to relative changes in sea level, in particular rise in global sea level and relative rise caused by natural or anthropogenic subsidence.

“It is clear that a free flowing river system creates natural fishery estuary,” said Gulf Seafood Institute’s President Harlon Pearce. “The channelization of the Mississippi River and salt water intrusion from the ever increasing number of petroleum industry canals has reversed the land building effect of a free flowing river system.”

“We are looking carefully at the Wax Lake and Atchafalaya deltas as models for building new land and preserving some of our coastal marshlands,” said Harry Roberts, director of the Coastal Studies Institute at LSU.

“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.”



Article by News Editor / Newsroom Ink
Article by NASA staff and Ed Lallo/Gulf Seafood News Editor