Rivers play a major role in the process of erosion - the constant wearing away of the land surface. Where a river's gradient is steep, the flow of water is faster. The gradient near the mouth is gentler and so the flow of water is slower.
Erosion occurs in three ways:- the impact of the water or hydraulic action, the abrasive action of the suspended material and by the dissolving of materials in the water.
Flash movie showing the effect of the river on stones
As the river gets bigger and is joined by more tributaries, the river has more water and so begins to erode a wider valley. It also deepens and widens its channel.
As the river gets closer to the sea, it slows down and starts to lose its load. This is one reason why deltas form at a river's mouth.
