I have been reading The Control of Nature by John McPhee in which he describes the efforts by the Army Corps of Engineers to keep the Atchafalya River from capturing the flow of the Mississippi above Baton Rouge, flooding Morgan City and leaving New Orleans high and dry. (Well, at least dry). (A good summary can be found on the web at http://www.ce.utexas.edu/stu/mcbraymc/ce385proposal.htm.) The key problem is that the Mississippi is continually filling its bed with silt, filling its current channel and thereby continually creating a need for a new channel. Attempts at flood control, in particular levees, exacerbate the problem by increasing the rate of sedimentation while decreasing the sedimentation area. The levees have turned the Mississippi into an “absurdly elevated aqueduct”, looking like “a large vein on the back of the hand”, so that tourists in New Orleans have to crane their necks, looking up to see the ships go by on the Mississippi.
An interesting situation in itself, but it led me to wonder if this were a unique situation. Are there other large rivers in the U.S. or the rest of the world that have been so extensively leveed, or for whatever reason, that they are in danger of shifting their channel? I don’t have much experience with large rivers but if my knowledge were limited by my personal experience I would be ignorant indeed. As always, in this situation, I turn to the teeming millions. Do any of you know of impending channel shifts? Is this a problem for most rivers? Only for really big rivers? What about the Nile or the Amazon? Or is it only a function of gradient – the slow, flat rivers all do this, while the high mountain streams never do?
Rivers change their path all the time. Typically, the river floods, which by definition means that its water level rises out of its banks. Some of the floodwaters may find another path and cut a channel that takes the place of the original riverbed. Alternately, the river may flood into and reopen an old channel, which once again becomes the path of the river after the flood.
I don’t know offhand of any other rivers that have been leveed as extensively as the Mississippi.
You heard it first here, folks. One day the Mississippi River will be a concrete banked sluiceway, just like the Los Angeles River. Sheer size is the only thing keeping it from becoming reality. Someday the losses from levee breaks will become so costly that it will actually pay to build concrete dikes.
Adding to the demand will be riverboat companies. It costs money to get a tow of barges hung up in shallow water. The Mississippi is constantly changing, making navigation difficult and expensive.
Anyone care to venture a year all this work starts?
runamuck, I think the work may have already started. There’s an awful lot of concrete lining the banks of the Mississippi in Minneapolis already. I don’t think that the bottom is concrete yet, but there’s a lot of concrete under St. Anthony Falls (from what I’ve heard, there would be no “falls” there anymore if they hadn’t done that–it was naturally eroding rather quickly.)
Rivers tend to “meander” near the end of their run (take a look at a map and notice how the Mississippi starts to wind back and forth in S-like curves after it gets past Illinois.) This is a natural process–levees may make it worse, but it still happens anyway. The Colorado is an example of a river that was old and meandering millions of years ago, until the plateau there was uplifted, rejuvenating the river. The faster water cut quickly through the rocky layers underneath, creating the Grand Canyon. The river still followed its old, meandering channel, though.
Heck the Mississippi is still a creek in Minneapolis;)
Down here in sothern Iowa,Burlington to be exact that old man river is channeled next to the west bank which is a bluff. The bluff on the Ilinois side about 20 miles east around Gladstone if you have an Illinois map.That kind of gives you an idea about the ancient, and not so ancient wanderings of that river.
As far as being completely concrete it will never happen.
The river is about 1 mile across here and when a levee breaks that can be 5 miles in a dangerously short time.
One of the reasons the lock and dams are necessary are sometimes there are rapids even on a big river. One of these rapids is south of Fort Madison near Montrose. The flatboats and steamers of old had a great deal of difficulty there.
Actually if it were concrete it would run so fast it would probably wash Mexico away;)
It is possible to have too many levies, weirs dams etc.
Australia’s Murray River is a glaring example of that. It rises in the Australian Alps and flows west to South Australia. It becomes a shallow system of three enormous lakes (by Australian standards) before reaching the ocean through a coastal sand dune.
Because of the many weirs that cross the Murray, it has now become a system of controlled ponds, rather than a flowing river. Much of its volume is reduced by large scale irrigation, which has had the disastrous effect of increasing salinity, as well as reducing flow.
The lack of flow becomes most obvious when the mouth consistently becomes blocked by sand and silt, because there is insufficient flow to keep it clear.