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I read this explanation about rivers: "As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders"

Why does the river erode laterally to the right side then the left side? Surely it would erode both sides at the same time and therefore the river would go in a straight line.

Could someone please explain more clearly how these meanders in rivers form?

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I'm under the impression you're not asking why a meandering river will keep meandering, but why even a straight river will start to meander in the first place. The first is indeed by erosion of the outside of the bends, and deposition on the inside of the bend, which is fairly easy to understand.

On the origin of meanders, this is a more complicated matter. Theoretically it might be possible to create a perfectly straight and homogeneous canal, with perfectly laminar flow, and in that case you'd expect equal erosion on both sides of the river. However, nature isn't as straightforward (if you pardon the pun), and rivers never start of perfectly straight, neither is the material ever perfectly homogeneous, and natural laminar waters are very rare.

Therefore, even if you would start with a straight canal, some parts of the walls are preferentially eroded over others, causing the canal to no longer be straight. This imperfection will then cause turbulence near the edges of the imperfection, leading to further erosion, thereby creating the seed of a meander.

Also, even if the walls were perfectly homogeneous, the slightest disturbance in the flow will cause the flow to oscillate leading to inhomogeneous forces on the walls and thus uneven erosion.

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  • $\begingroup$ I remember reading about an experiment where someone ran water down an inclined sheet of glass, and the stream formed meanders just from friction with the glass. $\endgroup$ – Spencer Dec 30 '16 at 4:24
  • $\begingroup$ I would be interested to see that, did the meanders move like in a river or did they just prove the point that slightly heterogeneous friction is able to bend the flow? $\endgroup$ – hugovdberg Dec 30 '16 at 8:11
  • $\begingroup$ Google search results extracted some tantalizing text from this 1960 article. But I can't get past the paywall. $\endgroup$ – Spencer Apr 8 '18 at 23:19
  • $\begingroup$ You can experience somehow the same process when water runs down a upright glass e.g. rain on a window. The friction differences due to dirt or other reasons causes the raindrop follow a "meandering" path and never a straight line. $\endgroup$ – 3TW3 Sep 25 '18 at 17:42
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Meanders amplify themselves--the stream tends to erode on the outside of the meander (forming a cut bank) and deposit sediment on the inside (forming a point bar). Eventually, the meander will propagate so far from the stream's mean path that the stream cuts across it. Eventually, the old meander will turn into an oxbow lake and fill in.

As to why it erodes on the outside and deposits on the inside, the simple answer is that the water has to travel farther on the outside, so it flows faster and erodes more, while the water on the inside moves slower and deposits its sediment. A more complete answer involves vortices crossing the river bed carrying sediment from the outside to the inside.

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    $\begingroup$ "water has to travel farther on the outside, so it flows faster" - isn't it more to do with the momentum of the water? Inertia means any mass will preferably travel in a straight line, which means that water parcels will push up against the outside of a bend. $\endgroup$ – naught101 Mar 3 '15 at 1:01
  • $\begingroup$ I too drew confusion with that phrase too... it's not like it must maintain the same angular velocity as solid body rotation would. But naught's explanation though makes good sense (a.k.a centrifugal force)? The fact its a longer distance vs same potential energy change, and more energy is lost in the collision with the shore suggests to me that it would probably flow slower on the outside? $\endgroup$ – JeopardyTempest Sep 25 '18 at 19:08
  • $\begingroup$ Though mvr.usace.army.mil/Missions/Navigation/Navigation-Charts/… suggests the outside of bends is better for navigation. I'm guessing that might be mostly due to depth rather than flow rate. Would be interested to hear confirmation. $\endgroup$ – JeopardyTempest Sep 25 '18 at 19:10
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Straight rivers usually occurs on igneous or metamorphic rocks and rugged relief.

Meanders rivers usually occurs in plain surfaces . Plain surfaces usually are covered by sedimentary rocks or sediments (easiest to erode). In a curve, the water is stronger in the open side and tends to erode, in the other margin it tends to accumulate sediment.

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