This post has more info. Apparently, the rains may yet cause flooding.

This page has some background:

The Old River Control Structure

As time progressed, it became increasingly apparent that the Mississippi was diverting more and more of its flow down the Atchafalaya River. In the 1950’s, engineers observed that the Mississippi would soon cease to inhabit its current channel as the mainstream, and instead migrate to the Atchafalaya River Basin. The path by which the Mississippi would migrate was a small stretch of water, named the “Old River”, that connected the Mississippi to the Red River. Old River was formed when Captain Henry Shreve dug a shortcut across the the neck of Turnbull’s Bend in 1831. The Mississippi abandoned its old course and took the shortcut provided by Old River. As a result, the Atchafalaya River received more and more discharge from the Mississippi. Discharge was also increased into the Atchafalaya in 1840, when a 30 mile long log jam was removed from its headwaters by the state of Louisiana. This increased discharge caused most of the problems the Army Corps of Engineers would have to face.

In their study of the Atchafalaya River, the USACE was able to deduce several possible effects of the diversion. The discharge of water into the current Mississippi channel would decrease until it resembled a bayou. All the levees along the previous Mississippi channel would no longer be needed to prevent flooding. In addition, towns such as Morgan City, located within the current Atchafalaya flood plain would be swept away by the newly expanded river. An expensive levee system would have to be built along the Atchafalaya in order to preserve current standards of flood control. The old Mississippi channel would no longer be able to be used for navigation by industry without expensive and extensive dredging. Industry would lack the water it needed to perform many of its processes such as cooling and the dumping of wastes. Agriculture would suffer from the lack irrigation water, and cities such as New Orleans would suffer economically from the lack of trade and drinking water. The only thing the diversion of the Atchafalaya promised to bring to society was disaster, and legislators decided to prevent this disaster at all costs.

The Army Corps of Engineers was given the job of maintaining the current distribution of water between the Lower Mississippi and the Atchafalaya River channels (70%-30%). They did so by building the Old River Flood Control Structure which consisted of massive floodgates that could be opened and closed as needed at the entrance to the Old River. This structure was completed in 1963. In 1973, a large flood tested the ORCS to its limits. Huge scour developed underneath the large steel pilings which anchored the structure to the river bottom. The structure was almost swept away, and emergency concrete was poured into the holes as a kind of large Band-Aid. After the ’73 flood, the corps saw the need for a backup structure, and built the Old River Control Auxiliary Structure (ORCAS) to alleviate some of the pressure on the main control structure during large scale flooding.

Despite several close calls, the ORCS still manages to keep the Mississippi River in check. How long this will last, however, is a matter of opinion. The Army corps claims to have the situation in control; the Mississippi will not divert to the Atchafalaya as long as they are there to prevent it. However, what if the control structures necessary to prevent the Mississippi’s diversion to the Atchafalaya River were completely undermined and swept away during a flood such as the one in 1973? The ORCS has almost failed in the face of the Mississippi’s might before, and it could still do so. Can the Army corps withstand nature’s might indefinitely, or will physics and the Mississippi River win out in the end?

Researcher Raphael Kazmann at LSU suggested that the Mississippi would be the victor in the struggle of man against nature. In his 1980 study on the possible effects of the Atchafalaya diversion he states, “Probably the most important single conclusion reached by this study is that in the long run the Atchafalaya will become the principal distributary of the Mississippi River and that the current main-stream will become an estuary of the Gulf of Mexico…the final outcome is only a matter of time” (Kazmann 1).

In addition to the flooding problem, engineers now face problems caused by the lack of flooding. The channelization produced by the levees and control structures deprives natural wetlands of the sediments normally deposited during flooding. Wetlands rely on sediments from distributaries and flooding to counteract subsidence, the compaction of sediments under their own weight. Water flows faster in subsidized areas, and distributaries can rapidly expand into wide channels, causing wetlands to disappear under the influx of water. The coastal marshes of Louisiana provide a natural barrier against the erosion causes by the fierce storms which often come from the Gulf. Because of the loss of these wetlands, the Louisiana coast has receded several thousands of feet over the past few decades, and commercial fishermen have also been deprived of a ready source of income.

Most of the problems resulting from the levee system, including wetland degradation, stem from channelization. While the levee system could not be scrapped without a large financial loss, the USACE realized that diversion structures could help alleviate some of the problems caused by channelization. Diversion structures diminish some of the force of flood waters and the likelihood of crevasses (breaks in the levee) by providing flood waters with established escape routes. The first diversion structure, the Bonnet Carre Spillway, was built in response to the great flood of 1927. It was designed to discharge excess flood waters into Lake Pontchartrain and thence into the Gulf of Mexico.

The USACE has recently started to build other diversion structures whose main purpose would be to divert sediment-rich water into wetland areas in order to stop subsidence. The Caernarvon diversion structure, completed in 1991, was the first of these modern structures. It has significantly restored wetland acreage and wildlife in the area. The success of the Caernarvon diversion structure has encouraged the government to develop more of diversion structures. Construction of the Davis Pond Diversion Structure began in 1997. Further in the future is a possible third freshwater structure located on the Bonnet Carre spillway itself. The Bonnet Carre Freshwater Diversion Structure would divert river water into Lake Pontchartrain, and finally the Western Mississippi Sound. With the help of diversion structures, the wetlands of the Mississippi River Basin may be able to offset the effects of subsidence and coastal erosion.

As the year 2000 approaches, the future seems uncertain for the lower Mississippi. Many questions regarding its fate reside in the hearts of both citizen and legislator alike. When will the next record-breaking flood take place, and what will be its effects? No one can tell whether the capricious river will flood its banks for a final time and permanently send its main flow to the Atchafalaya. Will the mighty Mississippi winding past New Orleans be reduced to a bayou? How much wetland habitat will be lost to subsidization and how far will the Louisiana coasts recede? These questions remain unanswered. Much work remains to be done to counteract the damage caused by our attempts to control nature; it is up to us to see that matters don’t become worse.