Erosion involves the wearing away of rock and soil found along the river bed and banks. It is the breaking down of rock particles which are then carried downstream.
Hydraulic action - the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away. Abrasion - rocks carried along by the river wear down the river bed and banks.
Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles. Solution - soluble particles are dissolved into the river.
Rivers carry material downstream this is transportation. Solution - minerals are dissolved in the water and carried along in solution. Suspension - fine light material is carried along in the water.
Saltation - small pebbles and stones are bounced along the river bed. Traction - large boulders and rocks are rolled along the river bed.
Rivers need energy to transport material, and levels of energy change as the river moves from source to mouth. When energy levels are very high, large rocks and boulders can be transported. Energy levels are usually higher at the source where its course is quite steep.