Erosion is the process that wears away the river bed and banks. Erosion also breaks up the rocks that are carried by the river.
There are four types of erosion:
Hydraulic action – this is the sheer power of the water as it smashes against the river banks. Air becomes trapped in the cracks in the rock of the river bank and bed, and causes the rock to break apart.
Abrasion – when pebbles are picked up by the flowing water and smash against the river bank and bed, wearing it away.
Attrition – when rocks that the river is carrying knock against each other. They break apart to become smaller and more rounded.
Solution – when the water dissolves certain types of rock, eg limestone.