Back to people & env.People & Environment - Sedimentation
Rivers carry sediment. When a river enters a reservoir, the speed of the flowing water slows down and sediment can be deposited on the reservoir bed. Over a number of years, the sediment in the reservoir can build up, and reduce the space available for storing water. Some of the sediment held back in the reservoir would normally be carried downstream. If too much sediment is stored, the natural balance of the river downstream can be changed, affecting people, wildlife and plants as far away as the river estuary.
A build up of black peaty silt behind Bottoms Dam, near Manchester, NW England
Farming land, used for growing crops, can be deprived of silt and its nutrients that are normally deposited when the river floods. Nutrients are important for fertilising the soil.
When designing a dam, the quantity of sediment that will flow into the reservoir has to be considered. The reservoir is designed to reduce the amount of sediment deposited, and to maximise the sediment flow downstream.
The flow of water carrying the sediment can be controlled by carefully positioning spillways and outlet pipes and tunnels. Sometimes sediment is allowed to build up in the reservoir. Then periodically, it is removed. This can be done by letting water out of the reservoir through outlets at the bottom of the dam, so that the sediment gets flushed out. Sometimes dredging is used for small reservoirs, but this is an expensive operation.