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.