In the upper course, the river erodes its bed by hydraulic action and abrasion. As the river flows downstream it is joined by tributaries, increasing the volume of water, velocity and therefore its erosive power. This enables it to cut a deeper channel as it flows downstream.

Downstream, the channel becomes wider as the gradient becomes more gentle leading to less vertical erosion. By the middle course of the river lateral erosion becomes the dominant type of erosion. The channel becomes wide because of lateral erosion.

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In the upper course, the valley cross-profile is narrow and steep-sided. The river takes up most of the valley floor. In the middle course, the valley becomes wider due to lateral erosion. By the lower course, the valley is almost flat, consisting of a wide flood plain.

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A channel cross-profile is a cross-section of only the river channel.

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In the upper course, the channel is very narrow and very shallow. By the middle course, the channel becomes wider and deeper usually over 1 m. By the lower course, the channel becomes wider still and the channel is much deeper.

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A valley cross-profile includes the channel, the valley floor and the sides of the valley.

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