What Causes Meandering Streams to Downcut and Become Incised?

One day, while out walking in the woods on a brisk fall morning, I found myself not quite at the bottom of a meandering stream.

 It continued to flow by my right side and a small tributary (I call Juniper Creek) flowed in from my left.

I walked about 12 more hours that day with no turning back. I felt like I was hiking through one of those old science fiction movies where an atomic war has happened where everyone had to live underground in these vast cave systems.

 The only light is from torches and fires at strategic points along the way. As I walked, I began to hear the sound of water slowly flowing and dripping. 

The sound increased until I reached a large cavern that was about 10 meters wide and 20 meters long with a stream of water flowing down the middle of it at a constant rate.

 Then the water began to spill over into the floor below and form some sort of reservoir. The sound echoed off the walls and seemed even louder than before.

I knew that this was going to be my home for at least a few weeks because it was too late to go back now. 

When I looked ahead, all I could see were huge caverns continuing in different directions with smaller caves off to the sides.

What causes meandering streams to downcut and become incised meanders?

It seems that different sides of the stream have different tributaries becauses of local topographic irregularities. 

For example, if a stream meanders more on one side than the other, there will be higher water levels on that side of the stream- this should result in a head current at the midpoint (or cut-off point). 

The water would also spill over into these side caves and cause backwater flooding. This could lead to a concave or incised downcutting pattern at the downstream end of such a meander. 

It is unclear why exactly some upland tributary valleys are incised but others not.

1. Downcutting by a meandering stream:

A head current develops along the middle of a meandering stream as the channel downcuts (such as seen in this case study above, which is a hypothetical one). 

The head current is directly upstream from and in front of the downstream (incised) bend.

2. Cut-off:

The head current erodes the concave bank of the stream, forming a cut-off point. The midpoint is at a right angle to the incised bend, where all flow is blocked and must flow overland across the floodplain.

 This causes a small river to form in what would otherwise be a tributary valley.

3. Overland flow:

As water from the cut-off point flows out onto the floodplain, it slows down and spreads out as an alluvial fan. 

The amount of sediment carried by this overland flow depends on how much water is flowing through the incised channel at any given time. 

This in turn depends on the downstream water level and slope of the stream bed, as well as the amount of sediment in the water.

4. Downstream cut-off:

Once enough sediment has been deposited by overland flow, an incised channel will form in what would otherwise be another meandering tributary valley. 

This continues downstream until all meandering tributaries are cut off or become buried by sediment. 

The new alluvial channels then converge to form a single channel, which in turn converges into a main trunk. This main trunk river is now incised and downcutting is complete.

5. Overland flow into the tributary:

The combination of overland flow from the incised main trunk and backwater from the tributaries generate a steep floodplain, with both steep banks and high water-table/floodplain levels. 

This makes it much more difficult for plants to grow in this area, especially trees. This is why you see plants such as bamboo growing in large swamps or forests of sword ferns.

6. Meandering tributary, cutting off:

As the main trunk incises, it creates backwater which causes the summit of the main trunk to over-top its floodplain. 

The stream retreats downwards and continues to overtop its floodplain until it reaches a suitable location for deposition (e.g., where a large island rests in a shallow sea).

7. Headwater incised:

The headwaters of this stream formed due to the head current along the middle bend and headwaters of the downcurrent on both sides were lowered, resulting in two headwater streams (upper right in this diagram). 

As the meandering stream incised, it cut off its tributaries, resulting in more headwater streams. This process continues downstream until all meandering tributaries are cut off or become buried by sediment.

8. Overland flow from distant incised trunk:

In a large river system, cut-off can be caused by a main trunk that is already incised as well as an upstream incised trunk. 

Lowland streams that have retreated far enough downstream to become lowland lakes can then contribute to overland flow in the main trunk channel. 

This form of incision is common in the Amazon River Basin and other large river systems where floods are rich in sediment (usually due to uplift).