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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 compressed in the cracks in the rock of the river bank and bed and is released with explosive force causing the rock to break apart.
• Abrasion - When pebbles grind along the river bank and bed in a sandpapering effect.
• 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 rocks, eg limestone.

The process of weathering also breaks down rocks in river basins.