It becomes more rounded and smaller.

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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. 

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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.

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Hydraulic action

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Abrasion or corrasion, attrition, hydraulic action and solution or corrosion.

Find out more about river erosion.