After receiving the Arkansas and Red rivers, the Mississippi enters a birdsfoot-type delta, which was built outward by sediment carried by the main stream since c.A.D. 1500 It then discharges into the Gulf of Mexico through a number of distributaries, the most important being the Atchafalaya River and Bayou Lafourche. The main stream continues southeast through the delta to enter the gulf through several mouths, including Southeast Pass, South Pass, and Pass à Loutre. Indications that the Mississippi River might abandon this course and divert through the Atchafalaya River led to the construction of a series of dams, locks, and canals by the U.S. Army Corps of Engineers. Known as the Old River Control Structure, it was undertaken to prevent such an occurrence. Sluggish bayous and freshwater lakes (such as Pontchartrain, Grand, and Salvador) dot the delta region.
Regarding the delta, environmentalists and those in the seafood industry are concerned by the loss of 25–45 sq mi (65–104 sq km) of marsh a year; fish and wildlife populations are threatened as their natural habitat slowly disappears. The loss has been attributed to subsidence and a decrease in sediment largely due to dams, artificial channeling, and land conservation measures. Pollution and the cutting of new waterways for petroleum exploration and drilling have also taken their toll on the delta. Louisiana has enacted environmental protection laws that are expected to slow, but not halt, the loss of the delta marshes.