River Movement and Erosion

Some rivers do not reach the sea because humans take so much water from them or there is a dry season which significantly reduces the volume of water going into the river.

A river valley changes shape as it is followed from source to mouth. Where the water is fast flowing, different processes of both erosion and transport occur than in the places where the flow is slower. There are three different stages of a river, the upper course, where valleys are narrow, the volume of water is smallest, but the flow is fastest. The middle course is less steep but there is a lot more water that is still flowing quite quickly and finally the lower course where the volume of water is greatest and the flow is slowest. This is where least erosion takes place.

Moving water erodes rocks and surface deposits in several ways. Abrasion is the process in which the river's load smashes into the banks and the bed causing pieces to break off. Solution is the process of dissolving soluble materials from the load, the banks and the bed. Hydraulic action is caused by air being trapped in cracks in a river's banks and bed. Finally, there is attrition which describes the process of the load particles hitting each other and breaking into smaller pieces. In the upper course, there is more vertical erosion and in the lower course near the mouth, there is more horizontal (also called lateral) erosion.

The movement of the river transports the products of erosion by several processes – traction, saltation, suspension and solution. Traction is the process of large pieces of load rolling along a river bed. Saltation is the name given to the process of load bouncing along a river bed. Where smaller pieces of load are being carried in a river's flow, we call it suspension and dissolved material is carried 'in solution'. Occasionally, you will see another process mentioned - flotation, where less dense objects are carried along on the surface of the river.