Where does erosion take place in a meandering stream?

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Erosion occurs in many places along the way, but the most significant area is where the stream begins to change course.

This is where high energy flows tear away at banks and undercut them, eventually creating a waterfall.

Erosion also takes place along the other points on a meandering stream as well. The more evidence of water flow you see, such as a rifle or a plunge pool, then that’s typically an area where erosion has taken place.

But what about the place that most of us associate with erosion – the waterfall? What is it that leads to erosion, and is there a relationship between meandering streams and these areas of high energy flow?

Meandering streams are more susceptible to erosion than rivers flowing in straight lines because erosion agents have more room to work.

Erosion agents have more room due to the fact that meanders are deeper, so they can attack banks at greater angles. 

They also have more shallow sections of stream, whereas straight sections are all long or very short. 

The higher water velocity on a meander also allows for greater scour effect of the river bank material, which creates deeper plunge pools.

Here are the points : where is erosion concentrated along a meandering stream?

1. On the downstream end of a meander :

 Where water has caught up with the upstream end after flowing in a straight line for some distance .

There exists a high velocity zone which is also called an acceleration zone .

 Meandering streams slow down when they reach these areas, which allows erosion to take place, because much more material can be eroded away than what would happen on a straight river. These areas are also called subcritical flow.

2. On the outer bank of meanders, headward erosion occurs :

This is because outer banks are protected by the inner, concave bank of the meander. 

Therefore there is not as much erosion occurring on the outside of meanders as compared to straight sections of rivers.

However, as you travel upstream, the outer bank begins to straighten out as it approaches the apex of the curve, allowing for headward erosion to occur which causes widening of meanders.

3. Erosion also takes place along the other points on a meandering stream as well :

The more evidence of water flow you see (such as a rifle or a plunge pool), then that’s typically an area where erosion has taken place.

4. At the place where the meandering water flow slows down or subcritical flow takes place : 

It creates an area for scour along the upstream bank. 

This area allows even more material to be eroded away, thus increasing the amount of plunge pool that can form. 

The greater scour effect occurs because there is less resistance to flowing water due to the low energy of subcritical flow.

5. When the upstream bank material has been eroded away so deeply that it finds itself in the stream channel :

Which creates a plunge pool beneath it where additional erosion can occur when the water flows over this previously eroded material. 

The only way erosion can occur on a straightaway river, with its uniform flow rate, is if there are no erosive agents present at all. 

And that is not true anywhere else on a meandering stream.

6. Erosion can also occur at the downstream end of a meander :

Within the zone of slack water, the water is flowing more slowly and this creates a zone of relatively still water behind it, thus creating a wave-making zone.

 The waves overtake the slower current and slowly break upon the outer bank as they move downstream. This produces a large amount of turbulence which can further erode away material from the bank.

Total amount of erosion will increase as you go from minor to major meanders and then to oxbow lakes and then bifurcations. 

7. The smallest meander in a stream is the most erosion prone :

Meandering streams are often referred to as “weak” rivers, because they have less resistance to flowing water than straight channels. 

They also have far more material available to be eroded, because anything in front of them is easily eroded away. 

A river traveling in a straight line with no obstruction will have less material available for erosion since it does not flow around any obstacles or bends in the channel naturally.

 A river flowing in a straight line will also have less resistance due to the fact that it flows at higher velocity, so there are more erosive agents present for making a more scour effect of bank material .

Area where the stream changes course; and an area where energy and velocity is higher than any other point on a meandering river. 

However, because of all the water present in a straight section of river, much more material can be eroded away than any other place on a meandering stream.

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