W-5

The formation of levees and floodplains are linked and involve repeated flooding and the build-up of material during the period of flood. Under normal low conditions, the river is contained within its banks and so no sediment is available to form levees of the floodplain. However, during periods of high rainfall and discharge when the river has burst its banks, both of these features are formed.
 * __Landforms resulting from deposition__**

Video showing the lower course features of the River Tay - [] Video showing the lower course features of the River Severn - []

Floodplain and Levees – Formation diagram

As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment.When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain.

When the river water bursts its bank, the shallower depth of water flowing over the surface results in frictional and a reduction in speed of flow. This results in the loss of energy and therefore deposition occurs. The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as //Levees (click on diagram above).// Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.


 * Key terms:**


 * Floodplain:** the flat area adjacent to the river channel, especially in the lower part of the course. This is created as a natural area for water to spill onto when the river reaches the top of its banks


 * Levees:** raised banks along the course of a river in its lower course. They are formed naturally but can be artificially increased in height.