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Old River Control Structure
Mississippi River Control Structure At Point Breeze
Point Breeze, Louisiana

Old River Control Structure

• Structure ID: N/A.
• Location: River Miles 303.7 To 316.6.
• River Elevation: 24 Feet.
• Structure: Spillway And Diversion Channels.
• Structure Type: Concrete.
• Length: See Below.
• Width: See Below.
• Date Built: Projects Completed 1958, 1963, 1973, 1985, 1986.
The Old River Control Structure, located on a stretch of the Mississippi River near a location known as Point Breeze, is the single most interesting spot along the entire Great River. In most cases, geological changes take place on a geological timescale. But at this location, a battle rages every day between Mother Nature and the US Army Corps of Engineers.

Going back 1000 years, the Mississippi River and the Red River ran more or less parallel to each other through Louisiana, with the Mississippi running past New Orleans out to the Head Of Passes, while the Red River ran through the Atchafalaya Swamp between Lafayette and Baton Rouge and down through Morgan City.

In the 15th century, the Mississippi River developed an oxbow loop in an area where the two rivers pass very near each other. This loop grew until it crossed the path of the Red River. This made the Red River flow into the Mississippi, and the Atchafalaya River became an outflow channel of the Mississippi River. In the 1600s and 1700s, the Atchafalaya River outflow channel became clogged by a monumental log jam. That prevented more than just a trickle of the Mississippi River water from flowing out to the Gulf of Mexico via the Atchafalaya.

By the 1800s, the loop in the Mississippi became so exaggerated that the river nearly looped back on itself, with the two river channels only a few miles apart despite being a river path of over 40 miles. In 1831, Henry Shreve dug a canal to cut off the loop, shortening the Mississippi River. The upper channel of the loop dried up, but the lower channel continued to carry the Red River as it flowed into the Mississippi. This channel is called the Old River.

In 1839, local citizens burned the log jam in the Atchafalaya River, and over the next few years, the state cleared out the remainder of the log jam from the Atchafalaya. Now, the once nearly blocked Atchafalaya River started to carry larger and larger volumes of Mississippi River water down to the Gulf of Mexico. By 1890, the Atchafalaya was handling about 10% of the Mississippi River flow. The more water that the Atchafalaya carried, the more well defined its channel would become, and the more water it could handle. By 1920, the Atchafalaya River was taking 20% of the Mississippi River water, and that grew to 30% by 1950.

The US Army Corps of Engineers came to the conclusion that the Atchafalaya River would become the new main channel of the Mississippi within a few decades. The key fact is that the Mississippi River took about 300 miles to get to the Gulf, while the path down the Atchafalaya was less than 150 miles. That means that the downhill slope is twice as steep going the shorter path. While the Mississippi River often changed paths down to the Gulf, doing so now would be a major disaster for Baton Rouge, New Orleans, and a major part of the petrochemical industry in the United States. The result is that the US Army Corps of Engineers was given the mission and funding to prevent the Mississippi River from making this course change.

In 1958, the Corps built the Overbank Structure. It consisted of a levee to keep the two rivers apart, and a low dam and spillway that was above the normal river level. This spillway would allow flood waters to be diverted down the Atchafalaya, but keep the normal river flow within its banks. In 1963, the Corps completed the Low Sill Structure and closed off the Old River. The Low Sill Structure is a dam and outflow channel that allows the Corps to regulate the amount of Mississippi River water that is diverted down the Atchafalaya. The goal was to maintain status quo of 30% down the Atchafalaya, 70% down the Mississippi.

This worked for 10 years until an especially large flood occurred in 1973. The Old River Control Structure was pushed up to and maybe past its limitations. A scour hole developed under the Low Sill Structure, causing part of the structure to collapse. The Corps was able to dump rock behind the dam, narrowly preventing it from failing. Had the dam failed, the Mississippi River would have changed course that day.

Following the flood of 1973, the Low Sill Structure was repaired, but the damage was done, and the dam was no longer as strong as it once had been, and not strong enough to hold back the river. To buy time, the Corps began to increase the flow being diverted down the Atchafalaya until it hit 35% in 1975. At the same time, construction began on a new control structure, this one called the Auxiliary Structure. The completion of the Auxiliary Structure meant that in tandem with the Low Sill Structure, the Corps could now defend against any flood that they could foresee happening on the Mississippi River, and a 30% diversion flow was again put in place.

Two other modifications have been made to the Old River Control Structure since the flood of 1973. First, the US Army Corps of Engineers decided to make the Atchafalaya and Red Rivers navigable. As part of this project, a lock was installed on the Old River, allowing smaller boats and small tows of barges to cross between the Mississippi River and the Atchafalaya and Red Rivers. Adding the lock required building a lift bridge on Louisiana Highway 15 to allow automobile traffic to cross the lock canal. The other change was the addition of a hydro power project. Engineers noticed that the water level was as much as 5 to 15 feet different in height between the Mississippi and Atchafalaya Rivers. A dam and power plant was built just north of the Low Sill Structure. The 30% river flow that normally freely flowed through the Low Sill Structure and Auxiliary Structure was blocked and forced to flow through the hydropower project. This is the first and only hydropower generator in Louisiana.

For now, the US Army Corps of Engineers has the Great River under control. The question is for how long? Some say that the river is now locked into its channel, while others say it is just a matter of time before an even bigger flood completes what nearly happened in 1973. There is also a serious moral question of if we should even try to hold back the river. Yes, the river changing course would cause a lot of short term economic hardship. But the long term consequences of not allowing the river to change path is already becoming evident with the sinking of vast areas of land and the loss of wetlands as the ocean reclaims southern Louisiana.


Old River Control Structure
Traveling downriver on LA-15, the first structure that you pass is the Sydney A. Murray power plant. The photo above is looking south, with the Mississippi River to the left, and the flow to the right. The highway crosses on the right side of the structure.

The photo below is looking back at the hydro power project after crossing the bridge. This view is to the north, with the river to the right, and the flow to the left. The power plant is a recent addition, having been completed in the mid-1980's.

Old River Control Structure
Old River Control Structure
The upper photo shows the view from just north of power dam looking east towards the Mississippi. Up to 30% of the flow of the Mississippi River is diverted through the power plant and down this channel. The Mississippi River channel is visible in front of the line of trees in the background.

The photo below is the Overbank Structure. It is very much like a conventional dam. It has 73 gates, each 44 feet wide, for a total length of 3,356 feet (about 2/3 of a mile). The Overbank structure is normally high and dry. But if the river floods and rises over its normal banks, the overbank structure holds the river back. It would only be opened in the most monumental of floods.

Old River Control Structure
Old River Control Structure
Above, another view of the overbank structure. An overhead crane travels along a set of rails, one rail on the structure, the other on the bridge. When the structure needs to be opened, this crane travels the length of the structure, and is used to lift the gates into an open position. This view is looking south, with the river to the left.

Below is a view of the overbank structure floodway, looking towards the west (towards the Red River and Atchafalaya) from the north end of the overbank structure. This area is maintained as a wildlife refuge and has a lot of small ponds.

Old River Control Structure
Old River Control Structure
Above is the Low Sill Structure. This is a conventional water dam. It regulates the flow of water exiting the Mississippi towards the Atchafalaya River. It is the first line of defense in holding the Mississippi River back and preventing it from changing course. This is the structure that nearly failed in 1973. The gantry crane is used to open and control the gates. The structure has 11 gates, each 44 feet wide, for a total width of 566 feet. It was designed to hold back a water height of 37 feet, but after being damaged in 1973, it now can only hold back about 22 feet of water.

Below is a view of the channel that carries water from the Mississippi River to the Low Sill Structure. This view is looking east from the north side of the Low Sill Structure. The river channel is less than 1500 feet from the Low Sill Structure, the closest it will come to any of the structures in the Old River complex.

Old River Control Structure
Old River Control Structure
Above is the output channel of the Low Sill Structure. This view is looking west from the south end of the Low Sill Structure.

Below is the Old River Auxiliary Structure. This dam was built after it was determined that the Low Sill Structure could fail in a major flood. The Auxiliary Structure has 6 gates, each 62 feet wide, for a total width of 442 feet. The curved wall on the input side (the side towards the Mississippi) can be clearly seen on the right side. The left side is the output side that is towards the Atchafalaya River.

Old River Control Structure
Old River Control Structure
A view of the Old River Auxiliary Structure from the south end, looking north. The Mississippi is to the right, and the Red and Atchafalaya Rivers are to the left.

A view from the same location, looking west down the new channel that allows water to flow from the Old River Auxiliary Structure to the Atchafalaya River. This channel has far more rip-rap than the other river channels, meaning that it is more resistant to the whims of Mother Nature and Old Man River.

Old River Control Structure
Old River Control Structure
Above is the input channel to the Old River Auxiliary Structure that carries water from the Mississippi River to the dam. This channel was dredged in the mid-1980's when then Auxiliary Structure was built. This view is looking south-east. The river runs more or less parallel to this channel, so it is nearly a mile before they meet.

Below is a profile view of the Old River Auxiliary Structure.

Old River Control Structure
Old River Control Structure
Above, a close-up of the gantry crane used to open and close the Auxiliary Structure.

Below is the Old River. This is the path that the Mississippi took before it was cut off by Henry Shreve. It was the original path where Mississippi River water flowed to the Atchafalaya. This river channel is blocked at the highway, LA-15, which is visible in the foreground. The device that blocks the channel is an earthen dam called the Old River Closure. This prevents any water from leaking out of the Mississippi and into the Atchafalaya without going through one of the control structures. The photo is looking south, with the channel running east towards the Mississippi River. The Mississippi is nearly a mile down this channel. A side channel flows off to the right, which is the entrance to the Old River Lock.

Old River Control Structure
Old River Control Structure
Above the river channel going to the west of the Old River Lock. This channel connects the lock with the Atchafalaya River, and indirectly to the Red River. The large concrete pillar is the west end of the lock retaining wall.

Below is the US Army Corps of Engineers sign in front of the Old River Lock project office. This is the only structure in the area that is formally signed. Some other structures have names painted on them rather than using this type of sign that is standard at most other US Army Corps of Engineers projects.

Old River Control Structure
Old River Control Structure
Above is the Old River Lock, which allows small tows to move between the Mississippi River to the east and the Red and Atchafalaya Rivers to the west. This photo is looking east towards the Mississippi River. LA-15 runs along the top of a levee in this area, a levee that is part of the earthen dam that closes off the Old River channel. The Old River Lock is 75 feet wide and 1,185 feet long. The floor of the lock is 11 feet below sea level. The lock operated 1,331 times in calendar year 2005, locking through a total of 3,589 vessels carrying 7,378,000 tons of cargo.

The photo below is looking north at the lift bridge over the Old River Lock. This bridge allows tall tow boats to pass through the lock without having to build a high bridge. The river elevation is normally about 28 feet above sea level in this area. The Atchafalaya and Red Rivers are lower, so the output side of the canal is normally at about 12 feet above sea level. This is a 16 foot difference in water level. It is no wonder the Mississippi River is trying so hard to break out to the west in this location.

Old River Control Structure

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Authored by John A. Weeks III, Copyright © 1996—2016, all rights reserved.
For further information, contact: john@johnweeks.com