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 drag and a consequent reduction in velocity (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 Levées. Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.
Find out more:
See these Wikipedia articles on floodplains and on natural and artificial levées This article called "Raising the Bar for Levees" looks at the role of Levees in flood protection an idea we will come back to in a few lessons time when we look at the causes, effects and management of river floods.
Key Term Check
- Floodplain - the area of land around a river channel which is formed during times of flood when the amount of water in a river exceeds its channel capacity and deposition of rich silt occurs.
- Levées - a raised river bank (can be natural features formed by deposition or artificial structures built to increase channel capacity and reduce flood risk)