Growing Deltas

As we’ve previously covered (, the Mississippi River delta plain is disappearing. The land loss along the Louisiana coast has been attributed to several factors, but particularly to lack of sediment that could build new marshes. Two deltas in Atchafalaya Bay, however, are growing. Comparing these satellite photos from 1984 and 2014 clearly shows an increase in land area at both. They are located at the mouths of Atchafalaya River, a distributary of the Mississippi, and the Wax Lake Outlet. About 40% of Atchafalaya’s water is redirected into Wax Lake as part of an effort to reduce severity of floods.

Prior to the Mississippi River being heavily dammed, most of the water was heavily laden with sediments. Now, sand and silt are often deposited behind levees instead of flowing into the Gulf of Mexico. The water is also forced through narrow channels too quickly to deposit the remaining sediment once it reaches the mouth of the river. Instead, it goes into the deeper waters of the Gulf, well past any place that could support delta-building.

Atchafalaya River still carries sediments, both naturally occurring and dredged, into the bay at just the right speed to allow them to settle and build up in the shallow coastal waters. Vegetation then colonizes the sandbars thereby stabilizing them and keeping them in place.

Scientists at Louisiana State University estimate that the two deltas together added about 34 square kilometers (13 square miles) of land between 1989 and 1995. Hurricanes then helped strip away 2 square kilometers (1.2 square mile) between 1999 and 2004; years which also had no major floods to replenish what was lost.

The growing deltas don’t come close to replacing the land that disappears in an average year, and the 50-year forecast is grim. Scientists say the Louisiana coast could lose another 5,000 square kilometers (3107 square miles). Wax Lake Outlet and the Atchafalaya River may serve as models for how to rebuild the deltas.

- RE

Photo Credit: NASA