As a river flows along its course it undertakes 3 main processes which together help to shape the river channel and the surrounding valley. These processes are erosion, transport and deposition.
River erosion is the wearing away of the land as the water flows past the bed and banks. There are four main types of river erosion. These are:
Attrition - occurs as rocks bang against each other gradually breaking each other down (rocks become smaller and less angular as attrition occurs)
Abrasion - this is the scraping away of the bed and banks by material transported by the river
Solution - chemicals in the river dissolve minerals in the rocks in the bed and bank, carrying them away in solution.
Hydraulic Action - this is where the water in the river compresses air in cracks in the bed and banks. This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks.
Material may be transported by a river in four main ways: solution, suspension, saltation andtraction (see diagram). The type of transport taking place depends on (i) the size of the sediment and (ii) the amount of energy that is available to undertake the transport. In the upper course of the river there is more traction and saltation going on due to the large size of the bedload, as a river enters its middle and lower course there is alot of finer material eroded from further upstream which will be carred in suspension.
Deposition is where material carried by the river is dropped. This will occur when there is no longer sufficient energy to transport material. Deposition of material may result in the formation of distinctive features such as slip off slopes (on the inner bends of meanders); levees (raised banks) and of course the floodplain itself. Remember - it is the largest material that will be dropped first as it requires the most energy to be transported. eroded from further upstream which will be carried in suspension.
This animation looks at sediment deposition as a river enters a lake - look at what size material is deposited first.
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