Atchafalaya Basin Dynamics
The Atchafalaya Basin is located in south-central Louisiana and is bounded by the Mississippi River and Tributaries (MR&T) system levees. The basin encompasses approximately 374,000 acres of fresh marsh, bottomland hardwoods, cypress swamps, and open water. The basin contains the largest contiguous tract of fresh marsh in the state, a valuable national resource. The Atchafalaya River is the dominant feature within the basin.
The Atchafalaya River has served as a distributary of the Mississippi River since the 1500s (van Heerden and Roberts 1980). Although most of the Mississippi River's flow has been directed towards the modern Plaquemines-Belize delta, river sediment was deposited throughout the Atchafalaya Basin and declined in the 1950s, when most inland lakes and bays achieved a sediment-filled state. Sediment deposition then began at the Gulf of Mexico in the Atchafalaya Bay, resulting in an emergent delta in 1973. The Atchafalaya Delta has grown each year since 1973 to its present size of 11.3 square miles.
The lower Atchafalaya River Navigation Channel bisects the Atchafalaya delta. The USACE maintains the 20 foot by 400-foot channel to the Gulf of Mexico by dredging approximately 11,100,000 cubic yards annually. Increased channel depth has formed an efficient conduit for river sediment to the Gulf of Mexico, depriving the adjacent delta environments of sediment critical to the delta-building process. Some distributary channels in the basin have undergone a reduction in carrying capacity due to USACE activities. The MR&T project - the comprehensive flood control project for the lower Mississippi Alluvial Valley below Cairo, Illinois - has had a significant impact on the water and land resources in the Atchafalaya Basin. However, the USACE has also created several hundred acres of marsh through judicious placement of dredged material and encouragement of delta formation.
The Atchafalaya Basin contains the resources to allow for substantial land building capacity. The flow rate of the river averages 226,760 cfs, with an annual average sediment load of 88,223,000 tons. Two channels, the Wax Lake Outlet and the Lower Atchafalaya River, convey flow and sediment to the Atchafalaya Bay. The Wax Lake Outlet conveys 38% of this flow and 35% of the suspended sediment. Two young, active deltas are forming in the Atchafalaya Bay at the mouths of the Wax Lake Outlet and the Lower Atchafalaya River.
The Atchafalaya Basin is unique among Louisiana coastal basins because it has a growing delta and nearly stable wetlands. Land loss has been centered around direct conversion to open water due to construction of canals, subsidence, and erosion. Riverine processes, which are the building blocks that create and maintain wetlands, dominate the basin. Human activities such as navigation interests and mineral exploration are the primary factors limiting delta growth in the Atchafalaya Basin, as these activities limit the riverine processes.
Sediment accretion usually keeps pace with subsidence in this basin, enabling the wetlands to remain relatively stable, healthy, and productive. However, localized wetland loss takes place where riverine processes are limited and during nonflood years when river stages are abnormally low. Human activity is also shaping the evolution of the Atchafalaya Basin. The flood control features of the MR&T project define the quantity and entry location of flow and sediment into the Atchafalaya Basin. The Lower Atchafalaya navigation channel conveys the majority of flow and sediment beyond the Lower Atchafalaya River delta affecting delta growth. Oil and gas exploration creates canals through the wetlands and the delta, disrupting natural sediment