>>Dr. Doug Elmore: Now we're going to take a look at some of the environments on the
Mississippi Delta. These pictures will be from a field trip I took with a group of students
there several years ago. This first picture is of two of our boats we took out on the
delta, and we're in the interdistributary bay. You can see marsh surrounding the boats,
and the water depth is generally very shallow there, a couple meters or maybe 10 feet deep.
In the background you can see some marsh. And here are some students sort of trapped
in a marsh. You can see it's very muddy. It actually stinks like hydrogen sulfide. It's
an important depositional setting on deltas though. You get a lot of mud deposited on
deltas. This is actually on the backside of a barrier island, and it's what's called an
overwash fan, and we'll learn more about this in the virtual field trip to a barrier island.
What we're looking at here is a sand overwash deposit back behind the student, and the barrier
island is actually off in the distance. And then the beach is on the other side of that
barrier island. You also find swamps and bayous, as you see here, on the Mississippi Delta.
There are also other features on the Mississippi Delta. As you see here, some oil tanks. Hydrocarbons,
oil and gas, are produced from the Mississippi Delta and from offshore, and there's lots
of infrastructure, as you can see here, out on the delta. This is the levee in New Orleans.
This is the man-made levee, which keeps the Mississippi River from flowing into New Orleans.
You can see the Mississippi River on the left, and then New Orleans is off to the right.
Now let's talk about what's happening currently on the Mississippi River and Delta. And here's
this map that we started off with, but I'd like you to focus on some different things
now. Start here at this point and follow the Mississippi down through New Orleans just
south of Lake Pontchartrain out to the delta. Now look at this other river called the Atchafalaya.
From this point right up here, it goes down to Morgan City at the Gulf of Mexico. Well,
the distance that the Atchafalaya has to flow to get to the gulf is about half that of the
Mississippi. And it turns out that the Mississippi River has been trying to change its course
to the Atchafalaya for some time. Or the Atchafalaya has been trying to steal the flow of the Mississippi,
depending on how you look at it.
Before we go on and talk about this in a little bit more detail, I just thought I'd show you
a few quotes from a very wise man. This is Mark Twain, of course. I'm not going to read
these quotes; I'll just paraphrase them. He was very impressed with the Mississippi River
and was concerned that we may try to change the river, but the river will always do what
it wants to do. So keep that in mind as we're going through the rest of these slides.
In the last hundred years, the Atchafalaya has captured more and more of the Mississippi
River flow, as this table indicates. In 1900 it was about 12 percent. By 1950 it was 30
percent. And this caused a lot of concern. As a result, at a location called Old River
Control, the Corps of Engineers built some structures to prevent the Mississippi from
flowing down the Atchafalaya. The key structure here is the Low Sill Structure you see right
there. Notice there's a channel from the Mississippi into the Atchafalaya over here. There's also
a large overbank structure, shown right there. These structures, as I said, were built to
keep the Mississippi in its current course. This is a picture of the Low Sill Structure
taken several years ago. There are two large cranes that move along railroad tracks, and
these cranes pick up gates that allow water to flow through the Low Sill Structure. You
can see there are two sets of gates here. Here we are looking down at one of the gates.
The cranes would then pick up that metal gate in the center of the picture, and that would
allow the water to flow through.
Now can you tell which side is the Mississippi and which side is the Atchafalaya? Well, you'll
notice that the right side is higher than the left side. The right side is the Mississippi,
and it's 15 feet higher than the Atchafalaya. Without those gates there closed, the Mississippi
water would be flowing down the Atchafalaya. In 1973, there was a large flood on the Mississippi
River and Delta. And as a result the Old River Control was put under significant pressure.
This is a very dark photograph, but hopefully you'll still be able to see the key features.
The first thing to notice is the extensive amount of water. The Mississippi is on the
left, the Atchafalaya is on the right, the Low Sill Structure is right there. You can
see there's water flowing through it from the Mississippi to the Atchafalaya, and this
long thin line right there is the overbank structure.
Now in 1973, the Low Sill Structure almost failed. And this is a photograph illustrating
the Low Sill Structure during the flood. And you'll notice, you see the Sill Structure
here, and there are wing walls. There's one here. There used to be one similar to that
on the other side, but it collapsed. And the Corps of Engineers was very concerned that
the whole structure would fail. If it had failed, the situation in Southern Louisiana
would look very different than today. The Mississippi would probably have been flowing
down the Atchafalaya. After the Low Sill Structure failed, they decided they needed to reinforce
that structure but they also built another structure called the Auxiliary Structure to
provide more control. What do I mean by control? Well, during major flood events they would
open the gates on these structures to let more of the water go into the Atchafalaya
than into the Mississippi. That would then relieve pressure on the structures, and so
they wouldn't fail. So all of the water in the Mississippi would not go down into the
Atchafalaya.
Here are some pictures, more pictures of the Low Sill Structure. There you see the wing
wall that did not collapse, and there's the other side of the Low Sill Structure where
the wing wall collapsed, and they have a lot of rocks there piled up as added reinforcement.
This is a view of the overbank structure, which in one of those previous photographs,
there was water on both sides right up to the road. So what're the consequences of modifying
the Mississippi River and Delta? Well, a key consequence is land loss in Louisiana. Here's
a statement you may have trouble believing, but Louisiana is losing one acre of land to
the ocean every twenty four minutes. This is a major problem. There are several reasons
for this loss of largely wetlands, including human development. A key reason for the land
loss is due to deltaic processes and how we have modified the delta. For example, we build
levees on the Mississippi River to control flooding. As a result, the Mississippi does
not flood in the interdistributary bay area. Now remember when we talked about processes
that operate on deltas, so long as you're adding sediment, the delta is fine. But once
you quit adding sediment, there's always sinking going on. So you have a lot of sinking of
land due to compaction of mud, of the underlying mud around much of the delta.
Now when we build levees, we also channelize the Mississippi, so a lot of the mud and sand
is carried out into the deep Gulf of Mexico and does not build any new land. So both of
these processes, channelization and the levees, which are connected, contribute to the problem
of land loss on the Mississippi Delta.
Some scientists have argued that the mud and sand that is lost to the deep Gulf of Mexico
is a very valuable resource. Why? Well, it's very good soil, and it's not adding new land.
So it's lost to us, and it doesn't build new land that we can grow things on. We'll come
back to that point a little later. This little simulation shows you land loss on the Birdfoot
part of the delta, just part of the delta, from 1956 to 1993. It's quite dramatic. Of
course the blue is water, the yellow is land. You can see that it was dramatic change just
from 1956 to 1978. Here's a map that shows areas where there's been land loss, which
is red, or predicted land loss, yellow, for 2000 to 2050. And then it also shows land
gain, or predicted land gain, in green. And you'll notice that in most areas we have had
land loss, significant amounts of land loss in Southern Louisiana. There's only really
one place where we're seeing much new land added, and that's over here on the left side
and that's over there by the Atchafalaya where there's some building out of new land. But
the predominant thing that's happening in Southern Louisiana is land loss. And here
are some projections of what the Louisiana coast may look like in 2100. Look at the right
figure. New Orleans is basically surrounded by areas that are less than one meter in depth
and would probably be principally water. The only parts of the land—or the only land
that's around are the levees. So that's a very serious situation for Louisiana. When
you add sinking due to compaction and sea level rise, absolute sea level rise, you have
some serious issues, or New Orleans has some serious issues.
So here's another issue with New Orleans. In the background we have a map of New Orleans,
and then I have a little cross section here of New Orleans. And you can see New Orleans
is basically a hole. New Orleans is here, the different parts of New Orleans, and then
on one side we have Lake Pontchartrain and on the other side we have the Mississippi
River. Notice that both Lake Pontchartrain and the Mississippi River are higher, the
water level is higher than most New Orleans. So if those levees were to break, where would
all the water go? Into New Orleans, and the levees did break during Katrina, and as a
result you had flooding in many parts of New Orleans. Lake Pontchartrain is on the right.
The Mississippi is on the left. This is the levee on Lake Pontchartrain. And there's Lake
Pontchartrain with New Orleans back behind us from the levee. And then here's the picture
taken from the levee looking toward the street level in New Orleans. And remember the point
here is that both Lake Pontchartrain and the Mississippi River are higher, the water level
is higher than most of New Orleans. And so if the levee were to break, or to be breached,
there would be a major problem with lots of flooding in New Orleans, just as there was
during Katrina.
During Katrina, the levee at the 9th Ward failed and many houses were destroyed. This
is a picture from the levee toward the 9th Ward. There used to be many, many houses on
all those plots of land over there. They've only rebuilt one, at least in this picture.
And this was a major problem. Unfortunately the 9th Ward levee had not been well taken
care of, and it failed. Here's a picture of the new levee at the 9th Ward. You can see
both those pictures, and the students horsing around, supposedly trying to stop the flow
through some holes in the levee. But here's something interesting. Notice those houses,
they're built up on stilts. You think those people are confident that that levee will
hold? Well, think about it. If they're building their house on stilts, probably not.
So why should we worry about land loss in Louisiana? Why should we save Louisiana's
wetlands? Well, there are a number of different reasons. It's a major area for shipping and
ports. New Orleans is a major port. There's abundant examples of recreational and commercial
fisheries. And it turns out the wetlands are very important because that's where a lot
of the fish spawn. It's a crucial habitat for water foul, alligators, et cetera. And
maybe one of the most important reasons is that wetlands provide a storm buffer by producing
friction to high-force winds. So as a large storm like Katrina comes in, if it has to
move over wetlands and land, it'll reduce the energy so when it hits New Orleans it's
not as strong. But if you don't have the wetlands, you don't have anything buffering the wind
energy, so when it hits New Orleans, it'll be that much more powerful. So a number of
different reason to save Louisiana's wetlands.
So what're we doing about land loss in Louisiana and on the Mississippi Delta? Well, the government
is using diversions to artificially flood selected areas on the delta to add sentiment
and try to keep up with the compaction. This shows a picture of one of the diversions.
New Orleans is up here at the top. This yellow arrow shows where they take water from the
Mississippi and sediment and let it flood down into this area. It's called Davis Pond.
And this is a diversion where they're trying to add sediment and water to the Mississippi
Delta, particularly the sediment to compensate for the sinking. This is very expensive, and
you can only do it in certain areas. You obviously can't do this in New Orleans because you have
a city. You can only do it in an area where you don't have many people.
So what're the consequences if the Atchafalaya were to take over the flow of the Mississippi?
I'm not going to read all of these, but you should think about these because there are
some very important consequences of if the Atchafalaya were to take over the flow of
the Mississippi, economic ones, social, and political.
So what do we do? Some scientists have suggested the Mississippi should be allowed to change
its course because of all the mud that is lost to the Gulf. If it did switch, then a
new lobe and new land would be created. Other scientists suggest that we should allow the
river to change its course because it will do so anyway in a catastrophic way, and it
would be better to control the change. This is an important issue we're going to be facing
in the next 50, or maybe 100 years. You should think about what you think we should do on
the Mississippi Delta.