About the Atchafalaya Basin

A Limited History of the Atchafalaya Basin

Over the past 10,000 years or more, the Mississippi River has changed its path several times, ranging from the current location of Bayou Teche to today’s route past Baton Rouge and New Orleans. The Atchafalaya River runs down the middle of that traditional Mississippi floodplain and would probably be the main channel of the Mississippi by now, if not for the Old River Control Structure[s] near Simmesport.

Formerly the lower section of the Red River, the Atchafalaya River became much smaller after the Red joined the Mississippi in the 15th century. By the 19th century, the Atchafalaya was blocked by a huge log raft, as were parts of the Red.

In the mid-1800‘s, river engineer Henry Miller Shreve and others opened the rivers to navigation and allowed the Atchafalaya rivers to carry a lot more water. Construction of dams along the Red River limited the open flow of water and sediment above the junction with the Mississippi. Eventually, dams along the upper Mississippi would also limit the sediment down the big river.

Structural River Control

In the late 19th and early 20th centuries, huge floods along the Mississippi River increased the size of the channel and the carrying capacity of the Atchafalaya River until concern mounted that it might capture most of the flow and redirect the Mississippi again. Congress directed the US Army Corps of Engineers (Corps) to build a control structure that would allow restriction of the flow down the Atchafalaya River to 30% of the total flow down the Mississippi and the Red Rivers. The 70-30 split, as it came to be known, was approximately the breakdown of the flow in the 1950s and was thought to be a reasonable choice that would allow the Mississippi River to maintain its current route. The Corps completed the first elements of the control apparatus in the 1960’s.

In 1973, a large flood threatened to destroy the main control structure and the Corps began a redesign of the control system to include additional structure elements. The resulting “Old River Control Structure” that exists today was designed and built to allow as much as half of the combined flow of the Rivers to be diverted down the Atchafalaya during a major flood and to allow configuration of the flow to minimize stress to the structures.

More Levee Building

In addition to the control structures, levees were built outside the lowest area through the Atchafalaya Basin, roughly five to ten miles on either side of the Atchafalaya River, from Simmesport to Morgan City.

The construction of the levees and subsequent straightening of the Atchafalaya River main channel changed the hydrology and siltation of the Atchafalaya Basin, both inside and outside of the levees. The new “main channel” follows the Whiskey Bay Pilot Channel, and the previous channel through Butte La Rose is silting in. Some historic channels were cut off by the levees and others were intentionally closed at the river to force the bulk of Basin water to be carried by the new main channel and hence, to scour the channel wider and deeper so that the flood control mandate could be met. A larger channel would also provide for easier navigation on the River.

Two undesirable results of building levees were to increase the height of flooding inside the Basin and to remove some of the traditional drainage paths from areas adjacent to but outside the levees. Higher flood levels inside the levees forced eventual permanent migration of residents from small communities like Bayou Chene and Atchafalaya. Removal of drainage paths increased flooding in low areas outside the levees as rainwater in the upper reaches of the traditional Basin moved through a restricted path on its way to the Gulf of Mexico and the land subsided without annual replenishment from river sediment.

Sacrificing Water Quality for Flood Control

The closing of streams at the levees and the main channel also reduced the amount of water flowing through the cypress-tupelo swamps and, in many cases, forced water to flow back into areas from the south instead of entering from the north, as it had before the closures. The back-flow filling of swamps and lakes aggravated siltation and removed the natural flow of water through those areas. That flow had washed silt and decayed plant matter from the swamps in high water times, but after the changes, water tended to flow into each area, slow down and sit until the water level dropped, often draining only once in a flood cycle instead of continually as it had in the past. In the summer months, standing water reached high temperatures that worsened low oxygen problems and discouraged growth of many types of aquatic life.

One result of half a century of flow in the new patterns caused by the levees and stream closures has been the loss of a large percentage of the open and deep water areas inside the Atchafalaya Basin Floodway System (ABFS) – that area between the protection levees and from US Highway 190 to Morgan City. Hundreds of acres of open water in “Grand Lake” are now willow flats and stands of cottonwood because of the inland delta formation caused by the increased slit carried by the deep, straight channel being dropped in the wide, slower-moving lake. While the new land might be good habitat for some wildlife, the lack of deep water makes it harder for fish to flourish during low water and high temperature periods.

It’s Not Just Because of Flood Control

In other areas, relocated water flow, canal construction, and petroleum related activities have created continually wet impoundments that are totally devoid of cypress-tupelo regeneration (and of any other woody vegetation). Tree seeds can’t germinate and survive underwater.  Thus, many areas of the Basin will become open, albeit shallow, water when the relic canopy of trees succumbs to insect damage, nutria herbivory or storm damage and occasional tree removal. Forest researchers report that landscape in many places has changed noticeably during the past 30 or so years of conducting tree health studies in the central and lower Basin.  In the areas above Hwy 190, agricultural expansion during the 1960’s and 1970’s played a larger role in deforestation than in the ABFS, but the net result has been a reduction in the number of trees, the amount of canopy coverage, and the continuity of forest corridors necessary for the proliferation of natural wildlife.

The impact of all human activity in the Atchafalaya Basin cannot be underestimated. The canal construction mentioned above has played a big part in restricting natural flow through the swamps. Canals were cut for logging access, petroleum well drilling, pipeline and highway construction, and, to a lesser degree, for more efficient water transportation. Dredge spoil deposits from canal and pipeline construction, as well as the erosion and siltation caused by boat traffic, have closed additional small drainage paths that were once the lifeblood of the swamps.

What is the “Atchafalaya Basin”?

It should be noted that the ABFS, that area often referred to as the Atchafalaya Basin in official and unofficial references, is but a small portion of the historic Atchafalaya watershed and drainage basin. It does not even include a large portion of the Atchafalaya Floodways, those portions above Highway 190, or the rare, actively building deltas below Morgan City and at the Wax Lake Outlet to its west.

Southern Louisiana no longer contains any areas that could truly be called “natural” because manmade changes have affected all areas from sinking barrier islands and vanishing marsh to the concrete roads and canals of our major cities. The Atchafalaya Basin is certainly an example of the consequences of such changes. Most observers now believe that the Basin should be managed in such a way as to mimic as many as possible of the natural processes that created it before we assumed control.

Natural Selection

The entire historic range of the big rivers suffers from the changes made to the flow patterns in the 20th century, and many areas outside the levees, as well as those inside, raise serious concerns. The condition of the “Basin” is often ignored in the push to save our vanishing coast. Disregard for the natural environment is a widespread problem and many areas, including the Coastal Zone and the Atchafalaya Basin, require immediate attention if we are to maintain the support of natural systems in our fight for survival as a species. Even if we should survive, the loss of these important resources would negatively impact our quality of life and our economic base. Conservation of our resources is everyone’s responsibility.

Current plans for an integrated approach to coastal restoration, flood control, environmental repair and wild area management are long overdue in south Louisiana, but advances are being made. The Coastal Restoration and Protection Authority is in the midst of a review of the definition of the Coastal Zone and the relationships of the inland and coastal areas in protecting us from storms and sea-level rise. The Corps of Engineers is reviewing the operation of the Old River Control Structure as a tool for supporting coastal restoration, in addition to its flood control mission.

It is our hope that future decisions about management of the Atchafalaya Basin, the Mississippi River flow, and coastal restoration projects will be made with an eye toward maximizing the positive effects on all affected areas. The lessons learned from the modifications to the Atchafalaya Basin should not be lost in the rush to move sediment to the coast. Unintended consequences in the Basin have proved to be impractical to reverse because of the scope of the changes, so open discussion of the likely effects on intermediate areas is important before large projects are undertaken.


This document was prepared by the Friends of the Atchafalaya, a 501(c)(3) non-profit organization, dedicated to promotion of conservation of all Atchafalaya Basin resources and is approved for reproduction and re-distribution.

Charles R. Caillouet, Jr., President, FOA Board of Directors
mailto:foa at basinbuddies.org
For Atchafalaya Basin information, go to http://atchafalaya.info

Additional Information Sources

For additional historic information on the Atchafalaya Basin Floodway System,
read “Designing the Bayous” by Martin Reuss.

For information on Corps of Engineers activities in the ABFS,
go to http://www.mvn.usace.army.mil .

For information on Louisiana State water management in the ABFS,
go to http://www.dnr.state.la.us/Atchafalaya .

For information on cultural and natural resources in the traditional Atchafalaya Basin,
go to http://atchafalaya.org .

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