Erosion involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.
The four main forms of river erosion:
- 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.
As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.
As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion). The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope of sand and shingle
The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).