How a river works

June 1, 2011 9:49 AM

What We've done to the Mississippi River: An Explainer

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.

Also: Restoring America's Delta a film by Cornell University.
Previously on Mississippi Flooding

posted by Potomac Avenue (14 comments total)

Atchafalaya is a must read. I didn't find it too technical.
posted by JPD at 9:59 AM on June 1, 2011

Damn, I thought this was going to give me someone to vent my rage at.
posted by spicynuts at 10:19 AM on June 1, 2011

"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. "

Huh. Did not know about 'distributaries.' Learned quite a bit with that article, thanks for linking it. Considering I live next to a big river that floods every year, it's probably good to know.
posted by librarylis at 10:33 AM on June 1, 2011

In 50 more years, we'll (humans) will probably still be keeping the river in its present bed. Which will be higher in elevation, as New Orleans is lower. It would take some political will unknown to our country, presently, to allow the Mississippi to change course.

And I do not even know if it would be the right thing, given that an untold number of people, rich and not rich, have so much at stake, keeping it flowing through (over) New Orleans.
posted by Danf at 11:03 AM on June 1, 2011

For a post like this I feel obliged to link to the Mississippi River Basin Model in Jackson, MS. The largest hydraulic model in existence is now in decay. I just spent the weekend with my parents remembering a 1984 road trip that involved hopping the fence and trying to find where our house would be on the model.
posted by cgk at 11:15 AM on June 1, 2011 [3 favorites]

Does anyone else read passages like these and find themselves longing for a clear, well-labeled diagram?

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 [...] The navigation lock had been dug beside this monument. The big dam, like the lock, was fitted into the mainline levee of the Mississippi.

[...]

Ten miles upriver from the navigation lock, where the collective sediments were thought to be more firm, they dug into a piece of dry ground and built what appeared for a time to be an incongruous, waterless bridge. Five hundred and sixty-six feet long, it stood parallel to the Mississippi and about a thousand yards back from the water. Between its abutments were ten piers, framing eleven gates that could be lifted or dropped, opened or shut, like windows.

While 1,000 words from John McPhee may be preferable to those from many other authors, isn't this a situation where a picture is worth more still?
posted by kcds at 11:29 AM on June 1, 2011 [2 favorites]

What's the difference between an explainer and an explanation? Isn't the explainer the person who gives the explanation?
posted by Eideteker at 11:32 AM on June 1, 2011

There is a ton of good links and information in the other thread on the Mississippi flooding.

Does anyone else read passages like these and find themselves longing for a clear, well-labeled diagram?

I thought the same - the wiki page on the Old River Control Structure is somewhat helpful.
posted by exogenous at 12:09 PM on June 1, 2011

I feel compelled to repeat: Atchafalaya is necessary reading. That is one of the best pieces of nonfiction on any topic that I've read in a long time*.

I love the hand-colored meander-map in the Explainer link. It's from Harold Fisk's Geological Investigation of the Alluvial Valley of the Lower Mississippi River. Here are some more of his maps, stitched together to show a long stretch of the river.

The US Army Corps of Engineers has many more awesome maps & geological reports available online for the Lower Mississippi. The Old River Control Structure is on the Artonish map (download).

The last best piece being, of course, "Toxic Dreams" by Jack Hitt.
posted by ourobouros at 1:32 PM on June 1, 2011

Danf writes "In 50 more years, we'll (humans) will probably still be keeping the river in its present bed. Which will be higher in elevation, as New Orleans is lower. It would take some political will unknown to our country, presently, to allow the Mississippi to change course."

The Mississippi will change course eventually. Even dredging can't prevent it as the slope decreases the longer the river gets. You'd have to not only dredge the width of the river but also remove the delta being formed at the mouth of the river to prevent the river from eventually switching channels.
posted by Mitheral at 4:38 PM on June 1, 2011

I think more appropriate than "allowing" the Mississippi to switch into the Atchafalaya, is to seriously question the simplistic cost / benefit economics of the clearing, straightening, and the ongoing dredging of the Atchafalaya and Wax Lake that spurred the switch in the first place. Would we need Old River Control and the two or three Auxilliary structures if Shreve hadn't made his cut, and if the corps hadn't cleared the Atchaf of trees and dredged it open?

Given the funding, Louisiana and the MVN of the USACE are invested in creating River Diversions to re-introduce the river into the estuary within Louisiana. I flew over Davis Pond today. this will rebuild marshes, which are essential for securing critical United States' infrastructure and the city of New Orleans. This will also re-establish ecosystem services that underlie the "sustainable" sectors of our coastal economy, as well as an ecosystem unique to America and the planet earth.

But those of us at the end of the pipe also need the Corps and upriver States to reconsider the Levees-Only policy upstream--how much flood protection, water filtration, and other critical services can be re-established by re-planting the floodplain with wetlands? I've no illusions that the rich farmland the river creates will all be "re-wilded," but consider that less than 10% of the floodplain from Louisiana up into Missouri is forested, when these areas used to provide "flood storage" as the corps puts it.

http://www.flickr.com/photos/eustatic/5744521808/in/photostream
[stolen from the southern forests report]

We the united states could also consider a better farm bill that de-incentivized Tile Drain Agriculture in the corn states (but mainly iowa and illinois). These tile drains are known for causing the Dead Zone in the Gulf, but i imagine they also aggravate the force of the flood pulse by shooting water into the Ohio, instead of letting it fill up gradually. The EPA task force on Hypoxia has not had success in promoting the rebuilding of Wetlands for the reduction of nutrients into the rivers (partially because of the Bush years, yes), but perhaps if wetlands were framed as a flood control solution, they would get farther?

Anyways, thanks for posting that Cornell video, it rules!
posted by eustatic at 5:27 PM on June 1, 2011

A really hard to understand (and still controversial) part of the 2011 Mississippi flood is how difficult, but common sense decisions taken early on, like destroying the levees at Cairo, and much later, essentially closing the river in some areas, for days, to normal barge/towboat traffic, helped to manage the flood at critical points like Vicksburg, the ORCS, and Baton Rouge. Clear cutting of woodland plots in and near the river floodplain, and long term agricultural use of the levee protected bottom land greatly reduced the amount of large tree debris (snags) that in previous floods damaged spillway controls, and even the ORCS (in the 1873 flood). Stories of barges breaking away from reduced 20 barge tows were still daily local news in Baton Rouge, but no errant barges damaged the ORCS, as they did in the 1973 flood. Blowing the levees upstream, along with other early management actions, and changed land use pattern from 1973, did limit actions and damage downstream, and kept critical structures like the ORCS and the Morganza spillway, and the Bonne Carre spillway and New Orleans levees, from being overtopped/damaged beyond functional limits.

Moreover, improved weather forecasting, and some major luck in terms of continuing precipitation and drought patterns helped immensely with flood control measures. As nile_red and others noted in the earlier thread on Mississippi flooding, the relative drought in the Atchafalaya basin that preceded the flood surge probably reduced the flood damage quite a bit, as the unusually dry ground soaked up millions of gallons of flood water, where it most needed to be slowed and managed. We can't count on that kind of meterological good fortune in the next "100 year flood," and even though technology may further improve our forecasting and flood management capabilities by the time that next awful event appears, this flood's main lesson is that, eventually, we must gracefully cooperate with the inevitable, and not that we can infinitely engineer a river, or a planet, for our economic convenience.

In fact, I wonder, personally, if, when the final bills are totalled for the 2011 flood, whether we really "saved" anything at all. I suppose, given the value of the Ports of Louisiana directly dependent on the current Mississippi main channel, and the petro-chemical, agricultural and commercial installations along the last 100 miles of the current river channel, we came out ahead.

But in some future spring, I can imagine the math will be a whole lot closer, when the river is only a little higher.
posted by paulsc at 5:49 PM on June 1, 2011 [1 favorite]

helped to manage the flood at critical points like Vicksburg, the ORCS,

Great! Now there are orcs to deal with, too!
posted by mikelieman at 5:36 AM on June 2, 2011

"Great! Now there are orcs to deal with, too!"
posted by mikelieman at 8:36 AM on June 2

Heh. ORCS = Old River Control Structure.
posted by paulsc at 8:10 AM on June 2, 2011