Squiggly Rivers: Newsletter Challenge (01/24/06)

The speed of the water passing through the river bed is slower on the inside edge of the river's turn than the outer edge of the turn. Silt and mud suspended in the water precipitates out at a faster rate when water is flowing slower and deposits build up on the inside edge of the turn. As the process continues the turn gets sharper and sharper and accelerates the process.

The whole things starts with small eddies in the river or stream that create slower speeds in the river's current, which begin the process of silt deposits that forms a turn.

It seems to me that the correct answer is gravity. Water in a river flows until it fills a depression and then, when it comes to the lowest rim of the depression it spills over. This occurs over and over again and the lowest rim is never in a straight line with the source of the water up stream. Therefore, the stream meanders across the country side. Of course if the water flows into bedrock and has a softer material at the same level it will take the path of least resistance.

Have you ever watched water running down the windshield when you wash your car? The water doesn't run straight down - it meanders. It will do so on even a polished flat surface.

Not that I have an answer to the question - but it appears to me that irregularities in the surface are probably not the whole answer.

Yes, gravity is a player, as well as topography. Sediments carried in the water tend to settle out faster when water is running at a slower speed. Faster running water will tend to pick up sediment. Water speed (current) is not uniform across the cross section of a river's bend. Current is faster along the outer bank than the inside bank of the turn.

Since water seeks its own level and will run downhill (there is your gravity again), the river can only meander in one direction so far before it must reverse direction to continue on its way downhill, so it zigzags back and forth until it empties or evaporates. However, rivers can, over time, move quite a distance from their original position (path), unless they begin to cut deep canyons. However, even the Grand Canyon meanders and is controlled by the same effects of erosion and sediment precipitation. The amount of meandering is controlled by a number of things, including current speed, amount of precipitates in the water, ground type, and seasonal changes to the river, to name a few.

As far as water's path across the windshield, that is a good observation. Water has one of the highest cohesion effects of any liquid, so even a "perfectly" smooth surface like glass has some resistance. I would guess that the water's path is probably randomized by the slight differences in friction along the glass surface, but that is a different principle compared to the river's meandering.

Yes, gravity is a player, as well as topography... Current is faster along the outer bank than the inside bank of the turn. I'd say that the river does not take a straight path because of hills, even short ones, on the terrain. When the water passes an obstruction like a hill, it tends to turn toward it like air flowing over a truck trailer deflects around its edges into the space behind it.

Surface tension, turbulent flow, wind action and microscopic scratches all take their toll on the smooth flow of water on a windshield. It is possible to see an approximation of "straight" flow on a sheet of clean glass. Prop it up indoors at a shallow angle (30 degrees or so) and drop water on it (don't let the water flow onto it). As a drop starts to go down the glass, it will move in a straight line until the perturbing effects take hold.

For rivers, topography is a major reason they don't start out straight, followed by turbulent flow and erosion. Dig a channel in the garden as straight and smooth as you can, and allow water to flow through it for a while. Watch what happens!

I agree with this - there's a positive feedback effect which amplifies the twisting. But what surprises me about the answers is nobody has mentioned ox-bow lakes. I'm sure most readers know, but just in case - these are formed when a bend is so sharp it's like a hairpin and the river, in a flood condition, takes a short cut across leaving a bow-shaped lake cut off from the river. In UK it's about the only thing people remember from school geography lessons, and it's become a bit of a joke.

Yeah mate, Billabongs

Your point being....?

A Billabong is a body of water cut off from the main stream of a water course when the stream cuts across the short cut typically during flood time. Flood time in dry country such as Australia may be several years between events.

Thanks Emjay, I've heard of a billabong (as in Waltzing Matilda) but never before thought about what it is. I'm not surprised other countries have different words for ox-bow lake, possibly in US it's called something else. Any offers from anybody?

Has anyone considered the linear momentum of the water flowing in the streams? It takes a lot of force on the outer edge of a bend of a river to divert such massive amounts of water to another direction and this pressure and motion would increase the process of erosion.
I agree with the inside of the bends building up from sediment from slow moving water, but the outer edge would seem to have the opposite effect. Anyone concur?

I would agree.

Plus all that sediment suspended in the flow would act as a scourer - accelerating loop formation and creation of ox-bow lakes.

This form of meandering tends to be a flood plain phenomemon - on relatively soft land. Where the land tends to be more erosion/abrasion resistant, in upland areas (up the rocky hills and mountains), the wiggles are more likely to be caused by obstructions which don't wear away as quickly, so you tend not to see ox-bows on the sides of mountains.

From a aeroplane, I think you're more likely to see the meandering low land rivers, as they're usually wider.

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I think the summary reason is that the earth is not a perfectly smooth substance and that the water erosion properties exaggerate each of the imperfections - giving us wiggly rivers.

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I've never checked the accuracy of the following statement, but I have been told that the Nile is the only north flowing river in the northern hemisphere. I've checked various British rivers out and have failed to find one that flows south to north overall. Anyone care to confirm or refute this statement? Is the converse true in the southern hemisphere?

"I've checked various British rivers out and have failed to find one that flows south to north overall."

Just wait until the poles reverse. ;-)

What happens in the interim between pole shifts when we have no compass direction and no cosmic radiation protection (see aurora borealis).

The St. Johns River in Florida flows north. It starts in central Florida, and hits the ocean in Jacksonville.

The River Ob in Russia flows north into the Arctic Ocean. I'd be surprised if there aren't others in Russia, and how about Canada?

Even on a perfectly smooth terrain, wouldn't the coriolis force (gyroscopic effect from the earths rotation) cause a similar response of flowing rivers as it does on flowing weather patterns causing a tendency toward and eastward curvature? It would seem then that this would go so far before gravity would cause it to yield back the other way until the terrain allowed another eastward trend. Not that this is the only thing driving the meandering but one of several.

This is a good point. Rivers flowing south in the North Hemisphere would want to head westward and would if there was no friction and the Earth was a perfect sphere. It would be interesting to note if over millenia south flowing rivers tend to progress westward. Other localized forces would tend to be much greater, but the coriolis effect, although small, is continuous. Once a river starts to curve it curves faster and faster as stated in other responses. The water traveling to the outside of the arc is traveling furhter and therefore faster, eroding the outer bank, while the water traveling to the inside is slower depositing suspended solids and adding to the inner bank. The river can keep eroding and curving until it loops back on itself and creates a new path.

The water will take the less resistive path through the land. Gravity and the moons pull on the earth will also affect the sway of the river.

There's an isolated oxbow just north of Kansas City Airport.