Why do rivers deposit sediment?

AQA GCSE Geography > Physical Landscapes in the UK > River Landscapes in the UK > Why do rivers deposit sediment?

Why do rivers deposit sediment?

Deposition is when material transported by a river is dropped. Material deposited by a river is known as sediment. The larger the material, the higher the velocity needed to transport it. Therefore, when velocity decreases, the large boulders are the first to be deposited. Whereas finer materials, such as clay particles, are the last to be deposited.

Deposition locations on a river

Deposition occurs whenever a river loses energy and velocity falls. This can be when:

a river enters a shallow area (this could be when it floods and comes into contact with the flood plain)
at the base of a waterfall
on the inside bend of a meander
towards its mouth, where it meets another body of water.

Deposition leads to the formation of a range of landforms in the lower course of the river.

Summary

Process of Deposition
Deposition occurs when a river loses energy and can no longer carry its load, causing sediment to be dropped along the riverbed or floodplain.
Decrease in Velocity
When a river slows down — for example, after flooding, entering a lake or the sea, or flowing over a gentle gradient — its velocity decreases and deposition takes place.
Order of Deposition
Heavier materials such as boulders and gravel are deposited first as they require more energy to move, while lighter materials like silt and clay are carried further downstream.
Causes of Energy Loss
Energy can be reduced by a fall in water volume, increased friction, or when a river spreads out across its floodplain during flooding.
Locations of Deposition
Deposition commonly occurs on the inside bend of meanders, at the mouth of rivers, or where the river flows into another body of water forming deltas.
Impact of Deposition
Over time, deposition builds up landforms such as floodplains, levees, and deltas, helping to shape the landscape and create fertile farmland.