GEOSC 10
Geology of the National Parks

Controlling Rivers and Wrestling with Mud

Controlling Rivers and Wrestling with Mud

 

landsat image (left) and an airboat (right)
Left: Landsat image of the Mississippi Delta, November 14, 2018.
Right: Research using an airboat by Oak Ridge National Laboratory, US Department of Energy, Mississippi Delta.
Credit. Left, Landsat image (USGS) (Public Domain). Right, Airboat, Matthew Berens (ORNL, U.S. Dept. of Energy) (Public Domain)

Controlling Rivers

Unfortunately, the wetlands of Delta National Wildlife Refuge and Barataria Reserve in the Jean Lafitte National Historical Park and Preserve are disappearing at an astonishing rate, because of the indirect effects of human activities. Estimates are that every year Louisiana is losing over 100 square kilometers of wetlands (equal to loss of a square with sides more than six miles long). Whether the wetland birds will continue to stream north for generations to come may depend on how humans respond to the challenge.

The Mississippi Delta is a massive pile of mud and sand from the Rockies and Appalachians, transported by the river and dumped into the Gulf of Mexico. Long ago, the Gulf of Mexico extended much farther north into the heartland of what is now the U.S.; over the last 70 million years, the delta has grown southward from near Cairo, Illinois (up by St. Louis), until now the former embayment has been turned into a projection from the end of Louisiana out into the Gulf. There, the delta is as much as seven miles thick. If you have ever watched mud settle in a bottle of water, or if you have observed how your boot packs mud down if you step in it, then you know that, over time, mud will compact under its own weight or under the weight of anything placed on it.

As the delta grew into the Gulf over the ages, a natural balance was reached. The compaction that occurred during a year would leave a little space at the top, but the springtime floods would bring new mud to fill that space. Trees and other vegetation would grow up through the new sediment, or re-seed on top, and the system would continue, wildly productive and vibrantly green.

Wrestling with Mud

Humans don’t interact well with this natural system. Many people have settled near the river. Plants can grow through the mud of floods, but people don’t enjoy having their houses slowly fill up with mud. So, humans have built control structures. We built dams upriver, which trap sediment behind them, and which hold some floodwaters in check. Because large floods do happen even with those dams upriver, we also built levees along the river in its downstream reaches, great walls that hold the river in and attempt to channel the floods to the Gulf rather than letting the floods cover fields and towns and roads upriver. We also dredge the river, deepening it to carry the water—and shipping. Other channels have been cut through the delta for oil and gas prospecting and production. The great floods that shoot down the river then do not spread over the floodplain and the delta, and thus do not deposit fertile sediment to fill the space left by compaction of mud, but instead are piped to the Gulf through these human-made or human-deepened channels, carrying the sediment far offshore to settle in mile-deep water.

Way back in 1996, when the very first edition of this course was taught, we wrote:

"Today, much of New Orleans, which does lie on the delta, is well below sea level. A tanker in the river between its levees is higher than the playing field of the Superdome. Rainfall, and water seeping from the river, must be pumped out so that the city doesn’t fill with water. If the pumps were to fail, the city would become a lake. The city steadily sinks deeper, and the levees are steadily raised by the Army Corps of Engineers, as instructed by Congress, to keep the river caged. Meanwhile, the wetlands of the delta, unnourished by new sediment, are sinking beneath the Gulf..."

Students in Geosc010, and in many other classes at Penn State, learned what elected officials and coastal planners and students at other schools also learned, that New Orleans was a disaster waiting to happen.

In 2005, this sadly came true, when the powerful hurricane Katrina came ashore near New Orleans. Almost 1400 people lost their lives, damages exceeded $100 billion (that is more than $300 for each person in the USA), more than a million people were displaced from their homes (and more than half a million were still displaced a month later, often because their homes were gone). Where natural wetlands should have slowed the waves from Hurricane Katrina (which was not a really huge storm by the time it got to New Orleans!), the high waters of the storm surge roared almost unimpeded from the Gulf. Parts of the levees failed. The pumps failed. The city filled with water, as much as 20 feet deep.

A slideshow of the aftermath of Katrina

But the city has been (mostly) rebuilt where it was, the sinking will continue, the loss of wetlands will continue unless many things are changed, and the levees that have already been raised will need to be raised more. With the likelihood that the strongest storms will get stronger and sea level will rise in the future (we’ll revisit this later in the semester), the scene will be set for an even more horrific disaster at some future date. Many options are available, including restoring wetlands, filling parts of the city with debris or other materials, moving construction to higher parts of the city, moving out entirely, and more; it will be interesting to see how much of this will be done. But primarily, the donations and tax dollars from the rest of the country after the 2005 disaster were used to rebuild the city directly in harm’s way, with the knowledge that the rest of the country will once again foot the bill when disaster strikes.

Another story is being played out in this region as well. The river wants to leave New Orleans. The city has a love-hate relationship with the river, fearing the floods but needing the drinking water and the shipping channel. The river can harm the city rapidly by flooding, or slowly by leaving.

To understand this tendency of the river to leave New Orleans, note that especially large, muddy, flood-prone rivers normally have natural levees (which are much lower than the human-made ones). When a flood happens, the water spreads out of the main channel onto the flood plain, which is the flattish region of river-deposited mud next to the main channel. As the water spreads out into the trees or houses of the flood plain, the flow of the water slows, and the water drops some of its muddy load. Although some mud is deposited wherever the floodwaters flow, more of the sediment is deposited very near the river where the water first slows. Hence, the mud layer from a flood is thicker next to the river than farther away, forming a natural levee. Humans have raised these natural levees in many places.

Cross sectional diagram of a river, with natural levees. Explained in text.
Cross sectional diagram of a river, with natural levees that separate the river from the flood plain beyond. In some cases, the upper surface of the river may be higher in elevation than the flood plain even when the river is flowing at normal levels, as shown here.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

When we discussed reservoirs, we saw that the delta of sediment formed when a river enters a lake must build up as well as out, so that the river still flows downhill into the lake. The same is true for a river entering an ocean. The Mississippi River, with its levees, naturally dumps mud into the Gulf of Mexico, slowly building out and up, lengthening and raising the riverbed. After a while, the river is a bit like a log flume in an amusement park, following a long path to the Gulf; a break in the levee wall would allow a much steeper, shorter, and more exciting downhill trip. The recent history of the Mississippi Delta is that, roughly every 1,500 years, the main outlet of the river has broken through the natural levee, like a log full of park-goers breaking through a curve in the ride, and the river has then followed that new shortcut. But, as mud is deposited along that new shortcut, it builds out and up, lengthening until it is like a long log flume, and then the river breaks through its side again, someplace else. This break-build-break-build helps create the classic shape of a delta.

Old River Control Structure. Read the caption for details.
The Old River Control Structure at the juncture of the Mississippi River and the Atchafalaya River. You don’t need to know these details, but in case you’re interested... In this photograph, the Mississippi River enters from the left and curves away to the right in the distance. The Atchafalaya River meets the Mississippi at three points and runs off to the bottom right. Control structures (dams) at each of the three forks of the Atchafalaya prevent most of the waters of the Mississippi from running into the Atchafalaya. The design of the structures is intended to keep 70% of the water in the Mississippi and 30% flowing into the Atchafalaya. View The viewo the east-southeast. The control structures are located at river mile 315 on the Mississippi (315 miles from the Gulf of Mexico). On the left of the river in this photograph is Wilkinson County in the State of Mississippi. Concordia Parish, Louisiana is on the right.
Credit: Michael Maples, U.S. Army Corps of Engineers (Public Domain)

During the 1940s and 1950s, the Mississippi started to break out, into a side stream called the Atchafalaya River. To save the shipping channel and the water supply for New Orleans, the Army Corps of Engineers has used levees and dams, especially the Old River Control Structure, to allow some water to go down the Atchafalaya while keeping a vigorous flow in the main log-flume channel past New Orleans. During a flood in 1973, the Corps very nearly lost the Control Structure, and the river, when a giant whirlpool undercutting the dam came close to causing it to collapse. The task of the Corps is very difficult, taming immense natural forces as the system becomes more and more out of balance.

Video: How New Orleans Doesn't Work (2min, 26sec)

How New Orleans Doesn't Work
Click Here for Transcript of How New Orleans Doesn't Work Video

Here's a great tree. And the great tree is standing up on a bluff looking down on the Mississippi River, which sits down in a valley. And the river has a little natural levee-- this, we haven't gotten to human built ones yet-- and it sits down in a valley.

And then, over on the other side, there's another natural levee, and then you go up another bluff to the top. And the river itself sits down in its river valley like this. And it is flowing towards you, which is what the head of this arrow shows.

And the river is sitting on a few miles thickness of mud that have accumulated over the years as the delta has built out into the Gulf of Mexico. And whenever you get a few miles of mud, everything is sinking under its own weight. And that, in turn, means that the surface is sinking, as well.

Now, nature has a way of handling this surface sinking problem, which is that during a big flood, the water spreads out over the flood plain. And mud falls out of the water, and the mud makes a new layer. And so as the surface sinks, more mud is added. And so there's no net change in the elevation. And that's just fine, except for one little problem.

Over here you have a city. And you've built this big city. And you really don't want that flood coming into your city. So you just call up the Corps of Engineers and you say, make a big wall, and make sure that that flood is not going to get into my city.

Well that's just fine, except that doesn't stop the sinking, because the thing is going down. And so if you come back later, what you'll find is that the surface has moved down to a place like this, and your city has moved down to a place like this. I'm using a darker line so you can see where it's gone to from where it was.

And so you tell them to make the wall bigger. But meanwhile, the city is sinking even deeper. And you've gotten up somewhere way down here, now.

And things are getting really nasty, because at this point, there's a really big storm. And it manages to get just over the top, and it fills your city with water. And then you're underwater and you're very, very unhappy. And this happened to New Orleans. And the sinking is not stoppable, so if it's rebuilt, it is likely that it will happen again.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

An excellent account of this is given in John McPhee’s book The Control of Nature, 1989, Farrar, Straus and Giroux, New York, which may be a little out of date but is still fascinating, and shows that policy-makers and others were warned about the dangers in the area long before the disaster of the 2005 hurricane.