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

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

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