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 weirs are 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.

