There are four types of erosional processes in a river:
Abrasion: The erosion of the river bottom and the riverbank by material being carried by the river itself.
Attrition: The rocks and pebbles being carried by the river crash against each other, wearing them down to become smaller, rounded pebbles.
Solution: The chemical erosion of the rocks of the riverbank by the slightly acidic water. This occurs in streams running through rocks such as chalk and limestone.
Hydraulic Action: The water forces air to be trapped and pressured into cracks in the rocks on the bank of the river. This constant pressure eventually causes the rocks to crack and break apart.
Erosional processes animation
Rivers transport material in four different ways.
The amount of load being carried depends on:
- the volume of water – the greater the volume, the more load it can carry
- the velocity – a fast-flowing river has more energy to transport and can move larger particles
- the local rock types – some rocks (e.g. shales) are more easily eroded than others (e.g. granite)
As a river flows downstream the amount of load tends to increase and the size of sediments decrease.
Deposition is when the river ‘drops’ the sediment which it is carrying. This happens when a river lacks sufficient energy to transport the load it is carrying. Deposition is encouraged by several factors:
- a river carrying a large load, providing more material to deposit
- a reduction in velocity e.g. inside of a meander, on entering lake/sea
- a fall in the volume of water e.g. at times of low flow during a period of drought
Processes vary along the course of the river and as a result the shape of the channel changes downstream.