Oxbows, Cut-offs, Meanders, and Snags

So, how do you lose an entire large-ish boat, that sinks intact, for a hundred years or so? Well, if the water is muddy enough, and the place is remote enough, you just don’t see it. Especially if that river likes to wander all over the place at whim, or at least at what to most people looks like whim. Welcome to the Mississippi and lower Missouri Rivers, two streams that snake more than the reptile house at the National Zoo.

What makes a river take a certain shape depends on a handful of elements. First is the geology under the river – is there room to move? Yes, over time the stream will cut a channel, but if the slope of the land is low, and the surface is flat, and the surface is also loose (lots of dirt, or sand, or small gravel), then the stream will tend to take on a series of S-curves, especially if the stream gets a decent amount of in-flow. Something like the Republican River in western and central Nebraska doesn’t get much inflow, so it tends to be straighter than similar streams east of the 40″ rainfall line. Glacial outflow streams carry lots and lots of gravel and smaller sediments, so they wander as well, even though they may be over bedrock (if you dig far enough). Even the Colorado River (western, not Texas) cut bends through very hard material, because it carried a lot of sediment that acted as an abrasive. It had room to move, until it cut that rather deep canyon.

Because of all the sediment, when the flow of water slows down, the sediment eventually settles out of the water and onto the bed. This builds up the bed, and if it happens long enough, the river moves to the path of least resistance. If the flow of the river rises and falls, as used to happen (still happens to an extent) with the Mississippi and Missouri, then the river also dumps “excess” energy by moving into an S-pattern. The bends shift location over time. This is why, after big floods, the riverboats on the Mississippi and Missouri had to be very wary of snags and changes in the river’s location. The flow of water that had carried small islands, entire trees, huge glops of dirt and sand, wasn’t there any more, so the river had less energy, and dumped the load. Now the boats got to find those lumps, and trees, and avoid them. You didn’t want to be the first boat up the stream after a big flood. Sort of like being the first rafting group down the Grand Canyon after a big storm or high water. Surprise!

DeSoto Bend in Nebraska is no longer a bend. It is fields around an ancient island. The river relocated. Depending on physics, weather, and inner perversity, a river may build up natural levees and terraces, like the levee that keeps the Garden District in New Orleans above many floods. That neighborhood was built on a natural rise, for good reason.

If the river moves far enough, the bend might get pinched off and an oxbow lake forms.

https://www.geocaching.com/geocache/GC2R70K_etobicoke-creek-meander-for-now Source of image used under Creative Commons Fair Use.

Over time, the oxbow may dry up. Unless you look at the soil from above, or study soil-type maps, you’d never know that there was once a river there. Fly over the Mississippi on a clear day in fall or winter, when the crops have been harvested, and you can see the ghosts in the fields, the long-gone bends and oxbows.

Fair use from: https://revision.co.zw/meanders/
Fair Use from: https://laurasriverfeatures.weebly.com/meanders.html

Rivers wander. People try to keep rivers from wandering. This means dredging a channel, or building levees to constrain the wandering. Dredging can speed up the river, meaning it erodes more, or starts snaking more downstream of the deeper channel, or eroding upstream, or “Yes.” Levees can work, until you end the levee and the river starts dumping sediment and snaking more. All these mean that floods will spread farther below the dredged area or levee, unless the river decides to dump lots of sediment on the dredged area. This is called agrading, “building up the bed” as opposed to degrading “down-cutting the bed.” If a river has to be channeled, ideally, there will be areas nearby that are left as flood-zones, so that the river can release the energy and sediment that it needs to lose. And so that floods will happen in anticipated areas, and downstream floods won’t be as bad.

The problem is, when the Mississippi and Missouri and other streams were channelized, we didn’t know what we didn’t know. As much as I’d love to see those rivers revert to their old ways, I don’t think people are willing to sacrifice the infrastructure and good farm ground that has been developed along the streams. It’s like, oh, the plan to reintroduce certain fish to the Rhine and revert it to what it was before 1800s. You’d have to dismantle so many buildings, factories, power-plants and other things, and then tolerate floods and erosion and deposition and the mouth of the Rhine moving around and no more (or far less) shipping, and . . .

Eventually, the river is going to win. Rivers do that. The Old River Control Structure on the Mississippi is going to “fail,” meaning the Mississippi will route around it and shift back to an earlier channel and outlet to the Gulf of Mexico. Eventually, levees on other rivers are going to be overtopped so badly that they can’t be patched for months, or more likely years. Physics is on the side of the river.

As one older Midwestern farmer put it, “The river always flooded. That’s what rivers do.”


11 thoughts on “Oxbows, Cut-offs, Meanders, and Snags

  1. Interesting. Makes a good argument for not living close to a river, especially at close elevations…

    • The definition of “close to a river” gets awful squishy, awful fast– there’s a town not too far from where I’m at that has a sign about 14 feet high, next to the road, that says something like “flooding high point, 20XX” (I don’t remember the date, it wasn’t 18 though.)

      It’s Iowa, so flat to someone from Washington, but you can’t see a river anywhere– it’s actually a bit odd, I swear, this place is COVERED with rivers. It’s not in a big basin, there’s no obvious reason that flooding would hit there worse than anywhere else, but it was close enough.

      How many miles is “close to”?

  2. I’m torn between geeking out over how the silt and clay deposition over the oxbow process affects the hydrology, and laying down with a headache.

  3. and then tolerate floods and erosion

    :shudders: Yeah, seen some of the *little* versions of what happens when the Return To Nature types succeed in getting in the way of normal flood prevention– remember when the ACE was told they should focus on environmental stuff instead, and all that farm land went underwater because Flooding Is Natural?

    Makes me want to dump the “natural” twits’ property into the middle of the suddenly unprotected areas.

  4. If you want to see the results of meandering on today’s world, check out the state borders along the Mississippi from Cairo, Illinois to north of Baton Rouge, Louisiana. It’s enough to give a cartographer a migraine!

    • On those undulating shapes:

      It was probably in the late 50 or early 60s in a Scientific American article. Long rails were used to cut the clickety-clack of the rails by having very long tracks (several of the old ones welded or whatever) and shipped on several freight cars. There were occasional accidents. The shapes of the tracks in a field being shipped made shapes similar to rivers in flat country or what you have shown today.

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