Calling All Ecohydrologists

May 27, 2008 9:39 AM

What does a river look like from above, over time?

If you were to look at a river estuary from a satellite over a period of 100 or 1000+ years, what might it look like? A writhing snake-shape whipping at the lake/ocean? How much will it 'roam'? What might it do? I'm thinking more of flatland deltas where the terrain isn't terribly channeling.

and I would love to see any animations/movies/etc that would illustrate how a river moves over time. Do any of you know of anything like this?

posted by a_green_man to science & nature (15 comments total)

It would try to straighten itself out by cutting off its bigger curves, I believe. See oxbows.

IANAE(cohydrologist), but my husband is a pedantic civil engineer.
posted by SuperSquirrel at 9:50 AM on May 27, 2008

It may try to do this, but I think it's been shown that rivers naturally curve back and forth (something to do with the way erosion works at varying speeds and the tendency to feed back).

Here is a quote from Fermat's Enigma:

Professor Hans Henrik Stølum, an earth scientist at Cambridge University, has calculated the ratio between the actual length of rivers from source to mouth and their direct length as the crow flies. Although the ratio varies from river to river, the average value is slightly greater than 3, that is to say that the actual distance is roughly three times greater than the direct distance. In fact the ratio is approximately 3.14, which is close to the value of the number Pi, the ration between the circumference of a circle and it's diameter.

The number Pi was origionally derived from the geometry of circles, and yet it reappears over and over again in a variety of scientific circumstances. In the case of the river ratio, the appearance of Pi is the result of a battle between order and chaos. Einstein was the first to suggest that rivers have a tendency toward an ever more loopy path because the slightest curve will lead to faster currents on the outer side, which will in turn result in more erosion and a sharper bend. The sharper the bend, the faster the currents on the outer edge, the more the erosion, the more the river will twist, and so on. However there is a natural process that will curtail the chaos: increasing loopiness will result in rivers doubling back on themselves and effectively short circuiting. The river will become straighter and the loop will be left to one side, forming an oxbow lake. The balance between these opposing factors leads to an average ratio of Pi between the actual length and direct distance between source and mouth. The ratio of Pi is most commonly found for rivers flowing across very gently sloping plains, such as those found in Brazil or the Siberian Tundra.
posted by phrontist at 10:05 AM on May 27, 2008 [2 favorites has favorites]

Reminds me of John McPhee writing about New Orleans and the Mississippi
posted by tiburon at 10:09 AM on May 27, 2008

The book Riparia talks about the geomorphological patterns of channel migration. I can't remember if they have any time series photos of the same river at different time periods but this is something that can be inferred on the landscape in the form of channel scars and oxbows.
posted by buttercup at 10:24 AM on May 27, 2008

Well, typically a river will form meanders which grow until the river avulses and the meanders become oxbow lakes. Over time, small meanders become more pronounced. This is because the water flowing on the outside of a river bend has to travel faster than the water flowing around the inside of the bend. As a result, the outside of the meander erodes away, and sediment is deposited on the inside of the meander. So through time, the natural tendency of a river is to exaggerate any bends. The bend becomes more and more pronounced, and eventually, it becomes easier for the river to break through the land that separates the parts of the bend (often during a flood), forming a new, 'straight' course. The abandoned meander becomes an oxbow lake. I found a pretty good animation here.

We can also look at larger scale sedimentary features than just the course of the river. From a sedimentological perspective, rivers are like strings of sand encased in mud. Most rivers have semi-periodic cycles of flooding. During a flood, the river overtops it's banks (duh) and water is carried into the often flat floodplain around the river. During a flood, the water is usually laden with sediment, since a flooded river is moving quite fast. (The faster a fluid moves, the higher it's ability to carry sediment). As the flood recedes, water is left outside the banks of the river (sometimes trapped by levees); this water is now barely moving, and so it drops most of it's sediment onto the flood plain. Since the water wasn't moving too fast, the sediment is usually composed of mud and clay. Inside this large body of fine grained sediment, we find small, linear deposits of coarser grained material, usually sand and silt. This is the sediment that was deposited in the river itself. This tends to be larger material because the river has a higher ability to move sediment than any of the surroundings. You will, of course, find abandoned meanders, and old river courses as well. So the sedimentary deposits left by a river are sort of like a box of fudge swirl ice cream: the ice cream is the fine grained sediment, and the swirls are the multiple linear sedimentary features that represent the ancient river itself.

The last feature of the river is its delta. The delta does several interesting things. First of all, it builds out and up. When a river enters the sea, it slows down very quickly, as it encounters water flowing the other direction. Since the water is slowing down, it tends to deposit the sediment it has been carrying almost immediately. The further a delta moves into the sea, the finer the material it deposits, so you see a pattern where it deposits sand very near the entrance of the delta, silt a little further out, and muds and clays the furthest out of all. Over time the pile of sediment builds up. Several factors mitigate the buildup of sediment. For one thing, the small distributaries which deposit sediment change course. As they deposit sediment, flow in them slows, and eventually they need to find another course. Also, the delta as a whole subsides. As the pile of sediment grows, the sediments at the bottom of the pile are under an increasing amount of weight. This weight causes the sediment to settle, and expel water from the pores in between individual sediment grains. This subsidence allows for more deposition on top of the pile, the increased weight of which causes further subsidence....etc etc. A great example of this would be....N'awlins. The reason New Orleans is below sea level is that it was built atop a delta. The delta continues to subside, but there is no source of sediment being deposited on top of the delta, since the Mississippi river is heavily channelized.

Hope that helps. Also, apologies for all the wikipedia links-you're perfectly capable of finding them yourself-but they're all pretty competent summaries, so I included them.
posted by HighTechUnderpants at 10:24 AM on May 27, 2008 [4 favorites has favorites]

The term you want is "meander", and yes, it will writhe around. Google "meander + river" and you'll get lots of info.
Here's a nice explanation of why.
here's a time- progression satellite image.
posted by LobsterMitten at 10:25 AM on May 27, 2008

You might want to look at Prof. Adrian Bejan's Constructal Theory
posted by Comrade_robot at 10:28 AM on May 27, 2008 [1 favorite has favorites]

Pruned highlighted these outstanding maps of the Mississippi. Post 1 of 5, flickr set, flickr image.
posted by stuart_s at 10:52 AM on May 27, 2008

Here are Fisk's geologic maps of the Mississippi River valley, showing the ancient riverbeds. You can see that it roams quite a bit.
posted by oneirodynia at 10:57 AM on May 27, 2008 [1 favorite has favorites]

Hayes' "Up a Lazy River" in American Scientist (Nov-Dec 2006) is a beautiful account of the modeling and mathematics of meandering.
posted by Mapes at 10:57 AM on May 27, 2008

I think this website operates via Pony Express, but it does have historical photos of the Yellowstone River, and shapefiles, if you have access to GIS software.
posted by desjardins at 11:33 AM on May 27, 2008

There is a great article about this in "PhysicaPlus-- Online magazine of the Israel Physical Society" with excellent illustrations and very well-motivated and intuitive mathematics:

River Meandering and a Mathematical Model of this Phenomenon

Nitsa Movshovitz-Hadar and Alla Shmukler

"All the rivers run into the sea; yet the sea is not full" (Ecclesiastics 1, 7). But why is it that they all go meandering to the sea? Why do rain drops usually fall in straight lines, and a ball, when let roll freely on a steep slope, rolls down the shortest way, while rivers don't flow in a straight line? Is there logic and order in river meandering? Is there a mathematical model, which can predict a meander's route?

These questions and more are addressed in this paper.
posted by jamjam at 2:07 PM on May 27, 2008

Nice! I obviously don't know enough to ask the exact right questions and this helps me start on the path I was hoping for. I'll highlight some of these posts after I have a chance to digest a bit.

How about this: aside form those already mentioned, do any of you have any favorite watersheds that exemplify this phenomenon? or any particularly well-documented rivers that you would want to share?
posted by a_green_man at 11:20 PM on May 27, 2008

As a pilot, I am able to view mighty rivers from high above on a regular basis, and to note the seasonal changes in flow rates and patterns. One thing that stands out from the air is the flood plains. It's very obvious where the flood plains begin and end, and how incredibly irresponsible it is to build there.
posted by dinger at 12:31 PM on May 28, 2008

Looks like that's it, so thanks everyone. You've all just made me happier and better informed :)
posted by a_green_man at 9:41 PM on May 31, 2008