It becomes more rounded and smaller.

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Hydraulic action

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Vertical erosion involves the wearing away and deepening of the river bed. This is mostly by hydraulic action. It is most common in the upper course of 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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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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