River Channel Changes

River Channel Changes

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Describe and explain the changes that occur in a river channel as it moves from source to mouth.

All rivers have an upper, middle and lower course. The long profile of a river is basically the changes in gradient at these different stages in the river, every river is trying to achieve a smooth, concave, long profile.

The river begging in the upper course, normally in the hills and mountains. Here the river is smaller, the flow is fast and load and water volume are slow. The river erodes a v-shaped valley. At this stage the channel is inefficient as it has a large channel surface compared with channel surface area. There will be more friction as more water touches the river bed. A steep gradient is needed to maintain the rivers energy levels. The path of the river is fairly straight.

A river can erode the soil and rocks which form its channel banks. They erode in four ways Abrasion, Corrosion, Attrition and Hydraulic action. The main erosion processes that take place in the upper reaches are:

Abrasion or corrasion-This is when rocks in the sediment load which is being carried along by the river hit the rock materials on the bed and banks of the river. This is most effective if the river is flowing at high velocities like the upper course. This is the main process that causes vertical erosion.

Hydraulic action- This is the force of moving water. Loose sediment is most susceptible to hydraulic action. This happens at high velocities.

Corrosion-This is the chemical weathering of minerals in rocks in contact with the river water. The minerals in the rocks are slowly dissolved, eventually causes them to break apart. This process is most effective where there is fast flowing water and the river is not already saturated with minerals again this will therefore be more effective in the upper course.

Attrition-this is the process where the rocks in the sediment load erode by colliding with each other as they are carried along the river. The result is that the sediment load becomes more rounded and smaller in size. Even though this process takes place in the upper course the sediment tends to be larger and more angular here as attrition has not been acting on them for very long, however Sediment can be added anywhere along the rivers course.

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The main landforms formed in the upper course are.

Waterfalls, they are formed when a river crosses a layer of harder rock. This layer is eroded more slowly than the softer rock beneath it. Eventually a step in the profile of the river is formed with a plunge pool formed in the softer rock. As the layer of hard rock is undermined it will collapse and so the waterfall moves upstream.

This forms a gorge, a narrow valley with steep sides.

Transportation/deposition in the upper course. High velocities result in sediment being transported in the river, larger, heavier sediments can be transported due to the high velocity. Silt and clay also need high velocities to be transported as they are cohesive (stick together). Due to the high velocity there is little deposition.

In the middle course the angle that the river flows down is less steep, the volume increases when tributaries join the river and the load increases as the river has had time to erode. The velocity is slower.

The river becomes more efficient at this stage as less water is touching the river bed, reducing friction. The river starts to become wider and deeper.

The river now erodes laterally. The main erosion processes happening here are

Abrasion or corrosion (definition in upper course) as this is effective in high flow.

Attrition this process will still be happening, the sediments will now be smaller and more rounded as attrition has been happening for longer.

The main landforms formed in the middle course are meanders.

  1. The river is starting to meander. Erosion is greater on the outside of the bend, deposition more on the inside.
  2. Large meanders have formed
  3. The river cuts through the meander, leaving a straighter section and an ox-bow lake.

Transportation/deposition in the middle course, low velocities result in sediment being deposited, larger course particles are deposited rapidly as the velocity falls. Even when the velocity falls, fine particles, once set in motion can still be transported. Sediment transported under lower-flow velocities, as bed load, may become suspended load under high velocities.

In the lower course, the river slows down as the gradient is almost flat. The volume is the largest on the rivers course. And the river is most efficient as less water is touching the bed and banks.

The river is no longer eroding and there is minimal transport, most of the load is deposited as bed load.

The landform formed here are meanders (diagram in `middle course'). The meanders here are probably bigger than previously. The flat area of land by the river banks is the flood plain, the river is most likely to flood in this section.

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