Rivers


A river is divided into three parts, or reaches. They are the upper part, middle part and the tidal part. In each part you can find certain kinds of plants and animals that have become adapted to the conditions there. A few animals and plants may be found in more than one part of the river because they are adapted to survive under a wider range of environmental conditions.

Upper reach


In the upper reach of the river the current flows fast. The bed of the river is covered with large flat stones or pebbles. Small particles like most of the sand and silt are washed away but, in some hollows where the current flows more slowly, some sand may collect.

What plants grow in the upper reaches of the river?
Very few plants grow here because the current it so strong. It prevents mud collecting for plants to grow their roots. A river moss can grow on the surface of the stones. It has long stems with leaves that overlap and point downstream. If they pointed upstream the current would rip them from their stems.

What animals can you find on the river bed?
Although the stoney river bed is unsuitable for plants to survive, there is plenty of animal life to be found. Many small animals live under the pebbles where the force of the current is weaker. Even so, they have adaptations for clinging onto the rocks. They may have flattened bodies and claws in the ends of their legs, like the stone fly nymph, or they may have suckers like the leech. Blackfly larvae make a pad of silk which they use to hold them to a rock surface. Caddis flies make stoney tubes around their bodies. Most of these animals feed on smaller animals that are washed down the river, while the leech attacks passing fish and sucks their blood.
Two crustaceans that you may find are the crayfish and the freshwater shrimp. The crayfish is like a small lobster. The freshwater shrimp has a flat body which is curved like a crescent. This shape allows it to wriggle between the stones and keep out of the strong current.

Are all kinds of fish found in the upper reaches of a river?
No. Only fish, such as the trout, which can hide away in rocks, or swim against the strong current, can survive here. The bullhead is a fish about 8cm long. It can hide under stones or rest on the river bed and face into the current. The water passes easily round its large head and tapering body and does not push it away. While resting, the bullhead can change the colour of its skin to match the river bed so that it cannot be seen by predators. The trout has a streamlined shape that lets water pass over it easily. It is a strong swimmer and can hold its position in the fast moving water. The fish that live in the upper reaches feed on the insects and crustaceans they can find under the stones.

What birds are found in upper reaches?
The dipper and the grey wagtail. The dipper feeds by running or flying into the water. It can walk along the river bed and pick up caddis flies and stone flies with its pointed beak. The dipper's nostril has a flap to keep the water out of its lungs and its feathers are covered with oil to keep them waterproof. The grey wagtail feeds on insects which emerge from the water and fly into the air.


Middle reach

In the middle reach, the river is wider, deeper and moving more slowly than in the upper reaches. Small particles, like sand and grit, settle in some places on the river bed and provide a place where plants can take root. The water also cuts into the river bank and makes it collapse. This produces a vertical surface of soft sand in which animals can burrow, and a shore of soil and gravel in which plants can grow.

Do the same plants grow on the river bed and the river bank?
No. Each place has its own special conditions to which certain plants are adapted. On the river bed the current is strong and roots must develop a firm grip, otherwise the plant will be swept away. The current may change direction frequently as the water flows between the stones. The plants would have to produce a great deal of strong material to stand up to the changing pushing forces of the current so they have become adapted to the habitat by developing lightweight, flexible stems and leaves. These move to and fro with the water current and are not damaged by its changing forces.
By the river bank the current is very slow and pushes with little force. The plants that grow here can make enough material to give them strength to stand up to the current and grow shoots high out of the water.

How are animals in the middle reach different from those in the upper reaches?
The slower current means that the animals do not need to be so strongly adapted for clinging to survive. Snails can live here and feed on the surfaces of water plants. Large insects, like dragonfly nymphs, may move around in the water by the river bank and hunt for small fish. The sand and mud provide places where freshwater mussels can burrow and filter the passing water for food.
The water is warmer than in the upper reaches and holds less oxygen. All the animals that live here are adapted to survive with a smaller amount of oxygen. The blood worm may be found in water that is very low in oxygen. This small, red animal has a substance in it which quickly takes in any oxygen from the water and stores it for the worm to use. We have this same substance in our blood to help us get the oxygen we need. It is called haemoglobin.

What fish may be found in the middle reach?
The fish that live here are adapted for living in a weaker current than in the upper reaches. They swim less strongly and have a more rounded shape that is less streamlined. The bream and chub are two fish of the middle reaches. The bream has a brown body with a humped back. It has a forked tail and grows about 50cm long. The chub grows to about 60cm and has a broad head and large scales with black edges. Both fish feed on insects, crustaceans, worms and molluscs.
The middle reach is also the habitat of the stickleback. This fish grows to about 8cm long and has three spines which stick out on its back. The stickleback is adapted for many freshwater habitats but is not found in fast flowing streams or ponds full of decaying plant material.

What birds are adapted to the middle reach?
When a river bank collapses it may form a sandy cliff in which two birds can burrow to make their nests. These birds are the kingfisher and the sand martin. The kingfisher has a long, pointed beak for spearing fish but it also uses its beak to dig out a tunnel in the sand cliff. The tunnel may be up to a metre long. The female kingfisher may lay up to seven eggs in a bare cavity at the end of the tunnel.
The sand martin looks similar to the swallow and house martin that can be seen in towns in the summer, but has brown feathers. In the breeding season a pair of sand martins build a tunnel by diving at the cliff face and pecking out the sand with their small, wide beaks. When feeding, the sand martins swoop over the river and use their beaks to snap up any insects that are flying by.

Is the otter the only mammal found on a river?
No. Although the otter is probably the most well known river mammal, it is very rare. Other mammals are much more common. The water shrew is about 9cm long and has dark brown fur and a pointed snout full of sharp teeth. It has poisonous saliva which it uses to kill fish and frogs when it bites them. The water vole is about 18cm long and also has dark brown fur but has a more rounded face than the water shrew. It feeds on water plants, snails and insects. Both the water shrew and the water vole make burrows in the river bank.


Tidal reach

In the tidal part of the river the current is flowing so slowly that the very tiny particles in the water settle to the river bed. They form a thick layer of mud. This is teeming with worms, molluscs and crustaceans which feed on the remains of plants and animals that are washed down from the other parts of the river. As the tide rises and falls so the mud is covered and then exposed. At low tide huge numbers of wading birds land on the exposed mud and feed on the animals buried in it.

Why can such huge numbers of birds live on the mud flats?
The animals that live in the mud burrow to different depths so that all parts of the mud contains food for the birds. Each species of bird has a beak of a certain length. For example, the dunlin is a small bird with a short beak. It only probes just below the surface and feeds on the animals that are living there. The snipe has a longer beak and can probe more deeply and feed on animals at a lower level in the mud. The curlew has a very long beak and can feed even deeper in the mud than the snipe. All the birds can feed on the same mud yet not compete with each other for food.
The length of the legs also varies between the species and this affects where they feed. Short legged birds like the dunlin must feed on the exposed mud or in very shallow water. Long legged birds like the curlew can wade out into deeper water to feed.

Can river fish live in the sea?
The tidal part of the river contains salty water. Only three river fish can live in it. They are the trout, salmon and eel. The other river fish are only adapted to live in freshwater and die in salty water. The water at the river's mouth is not as salty as the sea because the freshwater from the river dilutes it. This water is called brackish water. Some sea fish, like the mullet, bass and flounder, can live in brackish water.