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Louisiana's Old River Control Structure and Flood Protection




As mentioned earlier, the Atchafalaya River is a distributary of the Mississippi, but it has not always been this way.  The two rivers have an interesting history that includes people trying to merge the two.  Let's take a look at their area of contact, called the Old River Area since, as we will see, one of the early links was called Old River.

  • BEFORE THE 15th CENTURY:  The Red River (RR) and Mississippi River (MR) were separate rivers, more or less parallel.


-15th CENTURY:  The MR turned west and a loop, later called Turnbull's Bend, formed.  It intercepted the RR, which became a tributary of the MR and the Atchafalaya River (AR) was formed as a distributary of the MR.



-BY 1778:  The entrance to AR was occluded by a log jam.

-1831:  Capt. Henry M. Shreve, founder of Shreveport and a world renowned river engineer, dug a canal through the neck of Turnbull's Bend, thus shortening river travel time.


-Over time, the north section of Turnbull's Bend filled in with sediment.  The lower half remained open and became known as Old River (OR) and linked the three rivers.



-1839:  Locals burned the 30 mi long logjam of the AR to the level of the water surface.  In 1840, the state removed the rest.  Periodic clearing efforts were required until the 1860s.

-As time passed, AR became deeper and wider and began to capture more of the MR.  During high water in the MR, water flowed west in OR and down AR.  When RR was high and MR low, water flowed down AR and east through OR.

-1880:  AR was now large enough that there was only occasional eastward flow.  AR continued to grow and capture more of the MR. 

  • -1953:  The U.S. Army Corps of Engineers concluded that MR could change course to AR bed by 1990 if it were not controlled.  This observation came from studies that monitored latitude flow over the years. 

Percentage of Latitude Flow Entering the Atchafalaya River




< 10.0









The decision was made to control the river so that year-round 70% of latitude flow would proceed down the Mississippi River and 30% down the Atchafalaya River. 


-1963:  The Old River Control Complex was completed.






  New channels dug include the outflow channel and the navigation channel, the latter having a lock to allow for the passage of shipping.  The following table shows that the Old River Control Complex is working as designed:

  • Percentage of Latitude Flow Entering the Atchafalaya River










31.8 (Partial control restored)


30.0 (Full control)


-1973:  A large flood partially undermined the Low Sill Structure.  If it had failed, the MR probably have shifted to AR and possibly not been able to be shifted back.


Flood of 1973 showing the flooded overbank structure.


Lowsill structure showing the collapsed wing wall.


Potential scouring beneath the low sill structure. Had this happened, the structure may have collapsed. In fact, deep pits were scoured on each side, but did not unite underneath.


Repair done to prevent future scouring.


-1985:  Construction begins on Sidney A. Murray, Jr. Hydroelectric Plant.  It is situated just north of the Low Sill Structure with affluent from the Mississippi River and effluent to the Outflow Channel.


-1986:  The Auxiliary Structure (see slide above) with its new channel opened to relieve pressure at other sites.


One of the reasons that people suggested that the Atchafalaya would eventually capture (that is, the main flow of water through our state would exit Morgan City instead of its present location at the mouth of the Mississippi) is that the distance is so much shorter and steeper to the Gulf via the Atchafalaya than the meandering Mississippi:

-ORCC to Gulf via Atchafalaya:  142 mi

-ORCC to Gulf via Mississippi:  335 mi

If the events of 1973, as described above, happened, how would life on the lower Mississippi and Louisiana coast change?  Would the present-day Mississippi River suddenly dry up?  Would a fisherman sitting by the river see it go glub, glub, glub, with fish flopping around in the mud?  Would ship traffic stop on the river?  Would there be any impact at all?  The following description of possible life after the change is excerpted from Kazmann and Johnson (1980:10-16):

In the aftermath of the huge floods that would cause the main flow of the river to jump to the Atchafalaya River, aside from the cost, anxiety, tragedy, and aggravation of dealing with massive amounts of water being in the wrong place, there would be lingering issues that would change the way of life on the lower Mississippi.  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.  Oyster reefs would be immediately destroyed, and would take several years to reestablish and become productive (no erysters!).  It would probably take two decades to adapt to the new environment around present day Morgan City.  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 protective levees of the Atchafalaya Basin would have to be upgraded to handle the new pressure from Spring flows.  And, oh my gosh, think of the negative impact on the crawfish supply! 

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 freshwater flow would not offset the tidal movement of the Gulf.  Can you imagine the cost of piping or trucking enough drinking (and flushing, etc.) water from north of Lake Pontchartrain to supply the needs of Greater New Orleans?  Can you imagine Greater New Orleans without water for drinking and sanitation?  Even when the water was just barely increasing in salinity, there would be severe damage to water heaters, fire sprinklers, fire truck pumping systems, and more.  The quality of our coffee!  As mentioned above, the fisheries (especially those associated with the freshwater 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.  Also, the sugarcane industry would have problems without sufficient fresh water.

All of this adjustment, and we have not delved into the intensity of impact on peoples lives during the crisis and the adjustment period.  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 lives back together.  Imagine the emotional strain to the population - people losing a lifetime of accomplishment.  This would be a tragedy of monumental proportions.  It would interrupt life much like World War II.  

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.  Prices of all Louisiana products (from the natural resources [fisheries, oil, gas] to industrial products [poly vinyl chloride, polyethelene, etc.]) would soar.  The interruption of the pogy fisheries would be very negative for such food industries as chicken, catfish, and hogs (see the last section of the notes).  New Orleans is one of the most important ports in the nation, and it would suddenly cease to function; all shipping and related industries on the Mississippi River would stop.  International trade would be further imbalanced.  The massive fertilizer business would shut down and the agriculture industry would falter.  

And what about the economy of south Louisiana?  For a period of time, all the revenue would dry up and tourism would collapse.   Even Mardi Gras would possibly come to a halt!!!   Only the mosquitoes would do well!  And probably the cockroaches and Formosan termites.  

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.  


What condition might potentially lead to this scenario?   Experts predict that the ORCC might fail if the snowfall between Saskatchewan and New York exceeds that of the winter of 1972-73.



For ages, the Mississippi River followed a similar pattern year-after-year.  As the water began to rise in the spring, the river would leave the depths of its channel and fill the batture (the area between the levee and the water) of the natural levee.  All backwater areas would slowly fill.  When non-native Americans came on the scene, they began to build levees that protected them from floods during most years.  Occasionally, higher water would result in flooding, so the local folks would build the levees higher.

After the great floods of 1927, the public mandated that Congress solve the flooding problems of the lower Mississippi River basin.  They did so by constructing a levee and "relief valve" (spillways and related structures) system:



In the slide above, all the dark lines represent levees that were constructed for flood control.

Each spring, as the water in the river rises, there is a sequence of events that occur that protect the lower river basin from flooding.

1.  The water begins to pool into a large back swamp area in Avoyelles and Concordia parishes.  This area is a vital part of the system because it holds so much water and gradually drains as the waters recede through the summer months.  If they are ever lost (through development), the lower basin will be in grave danger.

2.  As the backswamp fills, water rises in the river channel and covers the batture.  In the Atchafalaya, this occurs between the internal levees that are along the margins of the river above the latitude of Baton Rouge, then the water spreads to cover the entire floodway between the guide levees that extend southward all the way to the Atchafalaya Bay.

3.  The flow rates of the Mississippi and Atchafalaya increase, but remain in the 70%/30% ratio.

4.  If the predictive model used by the Corps of Engineers indicates that the above three steps will not prevent flooding, then they go to the human operated structures:

  1. Bonnet Carré Flood Control Structure:  When the water reaches a critical stage (it was originally designed to keep the Carrollton Gauge, located at the Corps of Engineers headquarters at the river on River Road, below the 20 ft mark - which kept the river 5 ft below the top of the levee), this flood control structure is opened to allow up to 250,000 cfs of water to flow into Lake Pontchartrain through the Bonnet Carré Spillway.  It has been operated 15 times (1937, 1945, 1950, 1973, 1975, 1979, 1983, 1997, 2008, 2011, 2016, 2018, 2019 [twice]), and 2020.
    When operated, only the flow rates needed to offset flooding are allowed.  This is regulated by the number of bays (the actual openings along the Bonnet Carré – a total of 350) that are opened, and for how long each is open.
  2. Here is what has happened in the past:

1937 48 285 203,571
1945 57 350 250,000
1950 38 350 250,000
1973 75 350 250,000
1975 13 225 160,714
1979 45 350 250,000
1983 35 350 250,000
1997 31 298 212,857
2008 31 160 114,286
2011 42 330 235,714
2016 22 210 203,000
2018 22 168 203,000
2019a 44 206 208,000
2019b 79 168 163,000
2020 28   90   90,000

Bonne Carre 1984  USACOE

The Bonnet Carré Spillway in 1984.  Note the guide levees on each side.  The control structure is the obvious straight line on the river side in the photo.  See the leakage of water running into the spillway?  Lake Pontchartrain is at the other end.

Bonnet Carre 1936 & 1969 deposition

Bonnet Carre open 1997

Bonnet Carré open in 1997, looking toward the river.

Bonnet Carre 1983

Bonnet Carré open in 1983, looking toward the lake.


b.  Morganza Control Structure:  After the Bonnet Carré Flood Control Structure is opened, and if the river continues to rise to the next critical stage, this structure is opened.  It shunts up to 600,000 cfs into the Atchafalaya River through 125 gates of the Morganza Spillway.  Since this is a very rare occurrence (it has only been opened twice, in 1973 and 2011), the rich soils of the spillway are allowed to be used for farming, especially for cattle and soybeans.  If it has to be used, the lessors are notified and, if they cannot remove their animals and crops, they lose them when the waters are released through the structures.


1973 60 42 194,000
2011 55 17 182,000

2019 It was announced that it would open June 2, but it was cancelled. 






c.  Fuse Plug Levee:  If all the above procedures are not enough to handle the flood stage of the river, the last resort is the Fuse Plug Levee.  This is an east-west running levee located between the west guide levee and the west internal levee along the Atchafalaya.  To its north lies the great back swamp area; to its south lies the West Atchafalaya Floodway.  It works much like a fuse in your car.  The fuse consists of a piece of wire that will tolerate an electric flow of a certain level (e.g., 15 amps).  If a surge of higher electricity hits the fuse, the wire melts before the surge damages the electronics of the car.  The Fuse Plug Levee is lower than the adjacent west guide west internal levees.  If the water in the back swamp is not contained by all the above steps (1-3 & 4a,b), then water begins to flow over the fuse plug levee rather than over adjacent levees where it would flood human habitations.  Once water begins to flow over the top of the Fuse Plug Levee, it quickly tears it down until it carries a maximum of 250,000 cfs.  This is designed to work on its own, but if extremely critical, it can be dynamited.  The Fuse Plug Levee has never been needed.

NOTE:  All of the above design elements are in place to ensure that a maximum flow rate of 1.25 million cfs safely passes New Orleans.  The levee system of New Orleans is designed to accommodate this amount of water.  To give the size of this flow context, if all of the maximum flow rate of the Mississippi River was directed into the Superdome, the Superdome would be filled in 100 seconds.  That’s a lot of water!

Below is the "Project Design Flood," a schematic of how the flood protection system is supposed to work.  This figure shows the maximum flow that the Corps of Engineers has forecasted.  It is called the Project Flood.  The numbers depict the maximum water flow in the river at different points without major floods occurring.  It is the 1958 plan, with flow rates updated in May 2011.