It becomes more rounded and smaller.

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

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

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