'Ol Man River Keeps Rolling . . . In A Different Direction

The main channel of the Mississippi River currently runs past New Orleans.
However, as discussed at America's Wetlands Foundation website this may not always be the case and, but for work undertaken by the U.S. Army Corps of Engineers beginning in the 1950s, the main course of the river might have already changed.

Over the centuries the Mississippi River has often changed course throughout its length.  In the 15th century the river did a loop left and intercepted the Red River (see top & center of map above) which became a tributary and as a result of this reconfiguration the Atchafalaya River (AR), seen in the center of the map above flowing parallel to the lower Mississippi, was formed as another tributary.

By the 19th century the AR had deepened and widened and under certain conditions flow went from the Mississippi to the AR rather than the other way around and by the end of that century the AR was no longer a tributary to the Mississippi and about 30% of the great river's flow was reaching the Gulf of Mexico by the AR with the remaining 70% still passing New Orleans in the main channel.

In 1953, the Corps of Engineers concluded that without massive flood control work by 1990 the main flow of the Mississippi would be to the AR which provided a shorter and steeper connection with the Gulf of Mexico than the New Orleans route.  The initial Corps work was completed in the 1960s (see schematic below):
After flooding in 1973 which partially undermined the sill structure, additional work was undertaken to relieve pressure on the junction of the three rivers.

However, it is considered inevitable that at some future date a Mississippi flood will overwhelm the control system and the river's main channel will flow via the AR.  The America's Wetlands site contains excerpts from a study examining the implications of the course change:

Instead of 70% flow down the lower Mississippi and 30% flow down the Atchafalaya, the percentages would probably reverse.  The Atchafalaya would be a rushing, raging river, even during the fall for a period of time until it scoured the channel and filled in the lower reaches so that the flow would diminish.  Morgan City would have to be relocated, as would other communities and many businesses, possibly including the massive infrastructure of the offshore oil and gas industry.  Fisheries would be altered measurably all across the delta . . . Additionally, pipelines, bridges, and the like that cross the Atchafalaya would be destroyed or rendered unsafe.  The ruptured natural gas pipelines would place stress on fuel supplies for energy companies, but they would quickly change to more costly fuel sources and have little or no interruption of service.  Imagine the traffic jams when and if bridges on I-10, U.S. 90, and U.S. 190 collapse (what about the railroads)?  All trans-state traffic would have to be rerouted to I-20 via I-55 through Jackson, Mississippi, adding up to 615 miles to the trip (not to mention time delays from the traffic jams). . . 
 

The lower Mississippi would still have a copious amount of water, but it would be slack compared to today.  Shipping could continue to be an important industry, but it would be interrupted for a time.  The slack water would allow (cause) the thalweg to fill in and stop deep-draft shipping.  However, after intensive dredging efforts it may be found that a 50 ft channel can be easily maintained because of the tremendous decrease in sediment.  New Orleans, possibly Baton Rouge, and all other cities and towns along the lower Mississippi would no longer be able to get their drinking water from the river.  It would become too salty, since the lower fresh water flow would not offset the tidal movement of the Gulf. . .  As mentioned above, the fisheries (especially those associated with the fresh water river) would suddenly change.  And what about the massive petrochemical industry corridor?  Aside from the impact on shipping, which they could weather over time, industry could no longer use fresh river water for thermo-electric cooling.  The saltier water would corrode all the pipes and related instrumentation.  Of course, industry would change to salt-tolerant materials, but that would be costly and time consuming. . .  

All normal routines would stop.  Businesses would be closed, as would schools, normal government, etc., etc.  Virtually the entire population would spend months and months just coping - just putting their and others back together. . . 

One can also imagine the impact on the nation.  Massive use of Federal dollars to protect and restore Louisiana=s infrastructure.  Loss of natural gas (there would be brown-outs throughout the eastern seaboard).  Commerce would be interrupted by restriction of travel and Louisiana=s inability to focus on supplying items traditionally demanded from her natural resources by the nation.
 

Long term, we would adapt.  Once the drinking and sanitation water issues were resolved, tourism would return.  Coastal erosion could be reversed on the west side of the present-day Mississippi River.  Shrimp, oysters, and other fisheries would probably flourish after a number of years due to new marshes being produced and nutrient rich sediments being redistributed.
This would obviously place a lot of stress on at least two generations of residents.  We would survive, but it would be a new Louisiana and Mississippi River delta.  

Well, that is certainly going to be a very interesting time!