Cross profile of a river valley

The cross profile of a river valley is the result of the interaction with fluvial, weathering and slope (mass movement) processes.

The form of a river valley is commonly described as V-shaped . In a V shaped river valley, a river can only directly erode the area of its channel. The downcutting (vertical erosion) of the river creates a slope, allowing weathering and mass movement to occur. If these processes are slow (e.g. in arid or semi-arid climates), or if the rock type is very resistant to erosion, the result will be a valley shape dominated by fluvial processes - steep-sided and gorge-like.

If the rock type is more easily eroded, or the climate allows faster rates of weathering, hillslope processes become more important, and the valley forms a more open V-shape. Humid temperate and tropical climates will encourage this type of valley shape.

At the valley base there is an important interaction between the river, and the slope processes. The sediment may collect at the slope base where it may be removed from the slope bottom by the river. This process is called basal removal . If the river is actively eroding its channel, basal removal will be efficient and valley slopes will tend to be steeper. If the river is not actively eroding, basal removal is slower and the valley profiles will be gentle.

This is most common in lower reaches, where lateral erosion dominates and basal removal occurs only along the hillslopes where meanders reach the valley sides. In other areas of the valley floor, material will accumulate. Lateral erosion, the movement of meanders downstream and infilling by flood deposits, create a flat valley floor. Meanders are most well developed in the lower reaches of the river where high discharges produce larger and more meandering channels.

Therefore, the energy of a river at any point along its course will be important. This will also vary in one place at different times as discharge levels vary. Most basal removal takes place during times when a river's energy is much increased. If the river has reduced energy - for example in times of low flow - material will collect at the slope base.