Delta build-up bucks the land-loss trend

Delta 1984
1984 – the deltas in Atchafalaya Bay. Credit: NASA/Earth Observatory
Delta 2014
2014 – the deltas in Atchafalaya Bay. Credit; NASA/Earth Observatory

NASA’a Earth Observatory has released a series of images from 1984 to 2014 showing the build up of new land at the mouth of the Atchafalaya River in Louisiana and Wax Lake Outlet, an artificial channel to the northwest.

The build up is in sharp contrast to the coastline elsewhere in Louisiana, where land is being lost at an alarming rate – a football field’s worth of land every hour. The delta has shrunk by nearly 5,000 square kilometres over the past 80 years.

By contrast, the new deltas are being built by sediment carried by the Atchafalaya River – a distributary of the Mississippi River.

Without human intervention, most of the Mississippi’s flow would  be through the Atchafalaya. Since the 1960s, however, only 30% of the Mississippi flows into the Atchafalaya River, with the rest directed towards Baton Rouge and New Orleans.

The images above show how the new land has built up to protrude into the Gulf of Mexico.

“We are looking carefully at the Wax Lake and Atchafalaya deltas as models for building new land and preserving some of our coastal marshlands,” Louisiana State University coastal studies researcher Harry Roberts told NASA’s Earth Observatory, which released the images.

The two deltas added a combined 34 square kilometers (13 square miles) of land between 1989 and 1995, for instance, but lost 2 square kilometers between 1999–2004, according to the LSU team. The land loss coincided with a period when hurricanes Allison, Isidore, and Lili battered Atchafalaya Bay and there were no major floods to replenish sediments.

The key reason that the Atchafalaya delivers sediment to the coast while elsewhere the Louisiana coastline is retreating is due to flow rates. The river flows at a pace that allows it to settle into shallow water and to maintain marshes.

In contrast, an extensive series of levees keep the Lower Mississippi’s water flowing in a narrow channel that whisks water and sediment past natural floodplains. 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.

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