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The formation section in the braided river Wikipedia says that very erodible soil causes braided rivers. Intuitively, a rising mountain range like the Pamirs would not be easily erodible.

The Yarkant changes so abruptly, I'm curious what special geological feature is at these straight/meandering-to-braided junctions.

The critical factor that determines whether a stream will meander or braid is bank erodibility. A stream with cohesive banks that are resistant to erosion will form narrow, deep, meandering channels, whereas a stream with highly erodible banks will form wide, shallow channels, inhibiting helical flow and resulting in the formation of braided channels.

Braid 1

Here's near the edge of the Pamir range.

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Braid 2

Here's another braided section in the Pamirs, far upstream.

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A source

There appears to be a glacier at a source of the Yarkant in Central Karakoram Park. If glaciers present/past explain braiding, why is it intermittent?

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Minimal braiding in the Rockies

Mountainous braided rivers seem rare/mild elsewhere, like here in the Rockies in Missoula.

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    $\begingroup$ I'm not expert on geomorphology, but the Wiki page says: "Braided streams occur in rivers with high slope and/or large sediment load." In my opinion, this is exactly what happens in mountainous regions. Your premise that rising mountain ranges are not erodible is not entirely correct. I would expect to be extremely erodible: Rapid uplift of rocks that are not stable in surface conditions, together with frequent below-freezing temperatures that work to break the rocks. The huge elevation gradients only assist in this. $\endgroup$ – Gimelist Sep 7 '15 at 11:17
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Braiding will occur in any river where two conditions are met: 1) a very high sediment load must be available, as is usually the case in a peri-glacial or post-glacial environment, and 2) there is an abrupt change from high energy rivers (steep hydraulic gradient) to low energy deposition with flat space on either side of the river. Go for a hike in any glacial or post-glacial mountains and you will see that rock hardness has little to do with sediment availability - there is always more sediment than the river can mobilize at any one time, even in extreme floods. The classic example is the Canterbury Plains on the east side of South Island New Zealand. Braiding is an inherently unstable and transient river configuration. Meandering is the river's minimum energy configuration. It is another chaotic process, achieved through helicoidal flow and superelevation on the convex side of a bend. It will occur anywhere the river is laterally unconstrained, and where the rotational kinetic energy, perpendicular to the direction of flow, is a significant fraction of the forward kinetic energy.

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Although the bedrock is indeed fairly resistant to erosion on the timescales considered for braided riverbeds, even the hardest rocks eventually erode because of all kinds of physical and chemical weathering. What you will often see in the mountains that at the most upstream ends of a mountainstream the streambed is dominated by huge rocks. These are only moved during heavy rainfall events or after a lot of time when erosion causes them to fall apart. The finer fractions are easily transported because of water and ice flowing over, and gravity pulling the sediment down the slopes.

At the bottom of the main valleys through the mountains the gradient is typically somewhat lower causing a lot of sediment to be deposited, leading to a fairly erodable riverbed. In this case it is therefore not so much erosion of the bedrock, but of the riverbed.

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  • $\begingroup$ It makes sense and is interesting to consider low and high gradient areas causing intermittent braiding. I wonder why the low grade in the Pamirs causes sediment sufficient to braid where the low grade of the rivers in Montana rarely result in braiding. Interstates and highways follow MT rivers because of their low grade. $\endgroup$ – 12345678910111213 Sep 17 '15 at 2:22
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    $\begingroup$ You should note that deposition of sediment is more dependent on the relative gradient (between different parts of the river) than the absolute gradient. If the gradient decreases the coarsest fractions are deposited only to be transported should either the gradient or flowvelocity increase. I'm not an expert on the local specifics, but it might be those interstates follow rivers that either don't receive much sediment as input or most is transported farther downstream. $\endgroup$ – hugovdberg Sep 17 '15 at 11:21
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    $\begingroup$ I would also wonder if the level-ish river valleys in MT have been settled and the rivers canalized. $\endgroup$ – cphlewis Sep 21 '15 at 6:41

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