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The lower reaches of the Mississippi River are being hit by record floods, as detailed on our In Focus photography blog. While there are thousands of news stories about what's happening, I found myself wanting basic knowledge about how the Mississippi works now. It's such a complex hybrid human-natural system with a deep and complex history that it's hard to know where to start.

While Humans vs. The Mississippi was most elegantly laid out in John McPhee's stunning story"Atchafalaya," we found ourselves wishing for a simpler tech explainer (or companion piece). How is a levee built? What's a revetment? What does the crucial Old River Control Structure look like? This explainer is intended to delve into these issues. 

What is the Mississippi River? It's not actually a silly question. The Mississippi no longer fits the definition a river as "a natural watercourse flowing towards an ocean, a lake, a sea, or another river." Rather, the waterway has been shaped in many ways, big and small, to suit human needs. While it maybe not be tamed, it's far from wild -- and understanding the floods that are expected to crest in Louisiana soon means understanding dams, levees, and control structures as much as rain, climate, and geography. From almost the moment in the early 18th century when the French started to build New Orleans, settlers built levees, and in so doing, entered into a complex geoclimactic relationship with about 41 percent of the United States. 

THE PROBLEMS

• The Mississippi floods.

• Here's how a river works. The sun shines on the earth, causing water to evaporate. That water goes into the atmosphere where eventually it gets agglomerated into storm systems, which the wind carries over the land until it gets released as precipitation and falls to the ground in some form. The liquid water wants to take the path of least resistance to lower elevation. According to the topology of a place, natural channels for the water develop. These are rivers. Rivers are connected together in vast networks of tributaries, which feed water into the main river channel, and distributaries, which pull water out of the main channel.

  As the rivers flow, they scour the channel in which they are flowing, creating "banks" to the river. Think of the process that created the Grand Canyon, but at far smaller scales. The banks hold the river under most conditions, but when somewhere upstream more water gets added to the system, the river can overflow its banks, submerging the land around it. That's a flood.

  Like all other rivers, pretty much, the Mississippi floods. Before humans built stable settlements, you could move away from the water, but if your town happens to sit near a river, you're stuck. The river is going to want to flood and you're going to want to stop it. That's the tension of the river. And the more you do to control the river upstream -- levees, say -- the more you cause trouble for people downstream. And almost everyone is upstream and downstream of someone.

  In the lore of the Mississippi, the floods of 1927, 1973, and 1993 earn special distinction. The earlier episode devastated the valley and set many of the processes in motion that created the current river management system. 26,000 square miles were inundated and 600,000 people displaced. The Flood Control Act of 1928, "the largest public works appropriation ever authorized," grew directly out of the disaster. The 1973 flood was largest of the post-management era. It nearly took down major systems that control the flow of water in the lower river and helped shape our current notions of what a worst-case scenario might look like. Scientists studying the disaster also found that much of the nastiness of the '73 episodecould be attributed to the changes humans had made to the river. The 1993 flood was themost costly Mississippi flood in US history.

  Below, you can see raw footage from the 1927 flood courtesy of the Internet Archive and Army Corps of Engineers.

   

   

• The Mississippi moves.
• Rivers change course, as you can see in the beautiful map below, which shows the river's meanderings. As rivers move from higher elevations to lower ones, the amount of energy contained in the water goes down. All the stuff the water had been carrying along in swifter waters starts to fall to the bottom of the river as sediment. In the lower portion of the Mississippi, the river moves very slowly. It's a broad, shallow river. 
  
  As the water moves basically south towards the Gulf of Mexico, it encounters bends. Differences in the forces at work on the inside and outside of a bend mean that the inner side tends to build up with with sediment while the outer side gets eroded away. 
  
  Sometimes, when a river overflows its banks, it can reach a new channel that provides a more direct pathway to wherever the river wants to go. If that happens, the new channel can become the main channel. If the Mississippi were allowed to do what it wanted, what is now the Atchafalaya River would become the new ending of the Mississippi. Again, in a purely natural world, that would be a six of one, half dozen of the other situation. But now human systems depend on the Mississippi remaining roughly as it was in 1900 when we started to build massive amounts of infrastructure. 
   
  
• The Mississippi is curvy.
• If you want to ship troops or timber down the Mississippi, you want to be able to go as close to straight as possible. The lower Mississippi, though, isn't straight. Because it's moving slowly and meandering, there are bends up and down it. Human beings have liked to cut channels between pieces of the river in order to cut down on the river miles in a given trip.
• The Mississippi silts up.
• The Mississippi tends to fill itself up with soil and sand. That problem's been exacerbated by excessive soil runoff from agriculture and other land-use change in the middle of the country. 

WHAT HUMANS HAVE DONE

John McPhee locates the beginning of the problem with the Mississippi way back at the founding of New Orleans. By 1724, a decree to build levees had already been promulgated. Things have only gotten worse since then. By 1812, there were already hundreds of miles of levee along the west bank. By the time of the great 1927 flood, there were 1500 miles of levees, and that was only the beginning. It was the Flood Control Act of 1928 and various addenda that would create and refine the Mississippi River and Tributaries project, which is managed by the Army Corps of Engineers.

The Corps made this film to highlight their role in shaping the river according to Congress' wishes. Ignore the soaring triumphant tone and just look at what the workers are doing to the river. This is how we changed the Mississippi forever.

Levees. A levee is a large earthen embankment that is used to contain the Mississippi River. They are largely just mounds of dirt covered in sod. The Mississippi has 3,500 miles of levees running its banks averaging almost 25 feet in height. There's nothing complicated about a levee. The main difficulty, really, is getting enough dirt to build these huge artificial hills. When a hole breaks in a levee, it's called a crevasse. Some get famous enough to have names.

Weirs. A weir is like a dam that is designed to be topped. That is to say, it's a structure that's built under the surface of the river's water to change the flow characteristics of the water. Bendway weirsare a popular concept for reducing the erosion and meandering problems noted above. Weirs angled upstream appear to cut down on the amount of erosion on the outer bank of the river by minimizing the secondary currents of the river spinning outward. Over 190 bendway weirs have been built in the Mississippi since 1989.

Dredging. Since 1930, the Army Corps of Engineers has been tasked with dredging the river to maintain a nine-foot deep navigation channel. That action takes place upriver using a boat operated by the Corps' St. Paul district called The Goetz Dredge. "Under optimum conditions, the dredge can pump as much as 1,300 cubic yards per hour as far as 1,650 feet and up to 800 feet inland," the Corps says. "Booster pumps are sometimes used in combination with the Goetz to pump material up to approximately 10,000 feet."

Revetments. Revetments are a way of strengthening the outer bank of a river to keep it from eroding. Over 360 miles of the river in the New Orleans district alone have been revetted. A popular kind of revetment on the Mississippi is basically a massive "concrete mat." In the video above, you can see one being built. In essence, thin concrete blocks held together by metal are created on land, loaded onto a ship, and dropped into the river.

The Old River Control Structure. The Mississippi really wants to flow down a different path than the one that it currently does. Rather than its current route, gravity is driving it to move down the Atchafalaya river bed to the Gulf of Mexico. Humans, having built cities and businesses with the current configuration, don't want this to happen. This is the main narrative of McPhee's outstanding New Yorker story. In essence, we block the river's natural path and divert the water into the Mississippi. That occurs at a former Mississippi tributary called Old River. "Rabalais gestured across the lock toward what seemed to be a pair of placid lakes separated by a trapezoidal earth dam a hundred feet high," McPhee writes. "It weighed five million tons, and it had stopped Old River. It had cut Old River in two. The severed ends were sitting there filling up with weeds. Where the Atchafalaya had entrapped the Mississippi, bigmouth bass were now in charge. The navigation lock had been dug beside this monument. The big dam, like the lock, was fitted into the mainline levee of the Mississippi. In Rabalais's pickup, we drove on the top of the dam, and drifted as wed through Old River country. On this day, he said, the water on the Mississippi side was eighteen feet above sea level, while the water on the Atchafalaya side was five feet above sea level." If the structure that keeps the Mississippi from becoming the Atchafalaya fails, it would be one of the largest catastrophes in American history. 

Spillways. Under flood conditions, the best way to take pressure off a place downstream is to let water flow upstream. That's where the spillways like the Morganza (seen above), which has been in the news, come into play. Opening up a spillway's gates creates an intentional flood somewhere in order to spare another place elsewhere. It's important to keep in mind that this is built into the system. It's part of how the whole system is supposed to survive record flooding.