How do rivers erode?
Fluvial erosion is the process by which a river wears away the land. The ability of a river to erode depends on its velocity.
Two types of erosion happen at different stages along a river. Vertical (downwards) erosion often occurs in the upper stages of a river whereas lateral (sidewards) erosion typical occurs in the middle and lower stages of a river.
There are four main processes of erosion. These are:
Abrasion, also known as corrasion, is when boulders and stones wear away the river banks and bed. Angular rocks that have entered the channel recently are useful tools of abrasion because they are more angular. Abrasion is responsible for both lateral and vertical erosion of the river channel (see below).
Attrition is when sediment particles knock against the bed or each other and break, becoming more rounded and smaller as you move down the river.
Hydraulic action is when the force of fast-flowing water hits the bed and banks and forces water and air into cracks in the bedrock. The repeated changes in air pressure cause the river bed to weaken. Hydraulic action causes vertical (downward) erosion in the upper part of the river and lateral (sidewards) erosion to the banks in the lower stretch of the river. This forms the outside bend in a meander in the middle and lower course. Lateral erosion is partly responsible for the migration of means across a floodplain.
Solution (or corrosion) is when acidic water dissolves rocks such as chalk or limestone.
The diagram below shows how erosion impacts the bed and banks of a river channel, causing vertical and lateral erosion.
Vertical erosion involves the deepening of the river bed. This is mostly by hydraulic action. It is most common in the upper course of the river. The energy that is left after overcoming friction leads to the channel getting deeper.
Lateral erosion erodes the banks of the river. This is more common in the middle and lower courses of a river.
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