Solution (or corrosion) is when acidic water dissolves rocks such as chalk or limestone.

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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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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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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.

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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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