Rivers can run for many hundreds of miles. The longest river in the world is the Nile in Africa. It is 6,700km long. The longest river in Britain is the River Severn, which is 354km long.

Types of river

crayfishGenerally speaking, in Britain you can find three main types of river;

- with both upland and lowland sections

- with upland sections only and flowing mainly westerly

- flowing only in lowland, found in the south and east of Britain.

Within these geographical areas, there is a variety of types which are determined by the type of rocks the river is flowing over. Along its route, the water in a river may pass over several different types of substrate (the rock below it); clay, sandstone, chalk, soft or hard limestone. Each kind of rock has an influence on both the water and the species of animals and plants that live there.

For example, the freshwater crayfish, a large crustacean, needs water with plenty of oxygen and a good supply of lime in the water to help build up its thick, outer skeleton; a fast flowing river running over chalk is ideal for this particular animal. A gravel-bottomed stretch of river will have a different group of plants to a stretch with a silted clay bottom.

Read More: Humans Influence on Rivers

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