Geology Quiz: What Causes Meandering Streams to Down Cut and Become Incised Meanders?

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Meandering streams and rivers flow more slowly than swiftly, so they cut down on their banks and become incised meanders. The uneven deposition of sediments on the inside of the bend leads to an outside bank with a concave upstream slope and a convex downstream slope. 

This is due to water traveling faster downhill than it does uphill thereby making deposits more on the downstream side. 

When meandering rivers wear away enough material, they change into steeper beds that cut down quickly resulting in sudden changes in river depth called rapids which cause turbulence. 

Consequently, these streams lose part of their ability to transport sand and soil downstream which eventually leads to slower erosion rates upstream that are consistent for this type of stream cutting pattern.

The answer is discussed here about what causes meandering streams to downcut and become incised meanders?

The reason a stream turns into a meander is that outside of the bend, the slope of the channel is not level in all parts. 

This results in deposition of sediment falling on one side and not on another, which causes the river to meander. This can only happen when there is enough room for deposition.

Here are some points discussed about Meandering Streams-

1. Meandering Streams and Meanders have low velocity flow.

The velocity of the channel decreases linearly with distance from the channel margin. The speed at which a stream curves depends on many factors such as depth, width, and slope of the channel, as well as stream gradient. 

The speed of a stream decreases with distance from the channel margin. This means that meandering streams slow down with deeper flow and erosion becomes more prevalent in them. In order to maintain meander shape over time, sediment deposition must be continuously produced in upstream reaches that are not subject to erosion.

2. Meanders require a constant slope for continued growth.

If the gradient of the river channel is too steep, the stream does not have enough silt and mud to create meanders. The banks would erode due to strong currents, leaving only bedrock in their place. 

If the gradient is too flat, these waterways would carry too much sediment and deposition on the outside of bends would be much greater than uplift on the inside of bends, leading to gradual straightening of meandering rivers till they lose their meander shape.

3. Meanders appear in low to moderate slopes.

Meandering streams occur in low to moderate gradients (1:100 to 1:5). They are common in the middle and lower parts of hillslopes and valleys where the gradient is low. 

This is because the meander bend radius is proportional to the channel gradient, so a very shallow gradient results in a large meander bend radius which does not allow for sharp meanders. 

On the other hand, very steep gradients result in rapid flow leading to steep stream banks which don’t allow for deposition on one side. 

4. Meanders are seen only after an incision due to tectonic uplift or stream capture.

If uplift is caused by tectonic forces, then the stream meanders will usually start downcutting into the incision. For example, if an uplift causes a hill to be formed in the middle of a large drainage basin, then the stream channel will cut down into this hill. 

However, if a meandering stream always cuts through meander bends and thus periodically encounters uplift, then its course remains unaltered till it reaches the end of its meander cycle.

In normal meandering streams without incision (called sub-parallel-channel), water can flow across both banks at right angles and still maintain their shape over time due to deposition.

5. Meanders can be created by meandering streams

In order for a meander to form, the stream must have an outside bank that is slightly more concave than the inside bank and a convex downstream slope. 

As a result, when sediment deposition is low, most of it cascades down the more concave side and these deposits build up until they make an outside-in curve. 

When deposition rate is high due to sediment supply from uplift or tectonic activity, then the stream bends until sediment saturation is reached where deposits are made on both sides of the channel.

6. Meanders are normally not symmetrical in shape.

The meander shape is determined by the relative rate at which deposition and erosion occurs on each bank. 

This is why a meandering river can be curved on one side while straight on the other side. The curve is maintained due to replenishment of deposits from uplift or tectonic activity. 

7. Meandering streams are eroded by their own banks.

As water flows across the stream bed, the banks erode and the shape of meandering channels is maintained. While erosion of the outer bank is substantial, this does not happen with inner bank erosion as far upstream deposition is high. 

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