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