Changes to food chains
Everyone in the village was fed up with the river. Each year, in winter, it burst its banks and flooded their homes. Carpets and furniture were destroyed; wallpaper pealed off all the downstairs rooms and the homes stank. A meeting was called in the village hall and it was decided to call in the river engineers to see what could be done.

Most rivers begin high in the hills. They form where streams meet. The water rushes down the steep hillside carrying sand, silt and grit with it. When the river leaves the hills it flows across a plain, which is almost flat. This slows down the river and makes it change its path. If you were in a helicopter above the river you would see it flows directly down the hillsides but when it moves across the plain it makes an almost zigzag path to the sea. The river curves one way and then almost back on itself as it moves across the plain. These curves are called meanders and are the cause of the villagers' problems.

When the river water slows down it can no longer hold onto the rocky particles it picked up in the hills. The sand, silt and grit are dropped to the river floor where the water flows most slowly. This happens on the inside of the curves and it is there that a shallow bank develops in which water plants can grow. When tree seeds land on the riverbank and sprout, their seedlings are not washed away. This means that many trees can establish themselves along the riverbank and hold its sand and grit in place. Although the trees hold the sand and grit in the riverbank firmly, animals can burrow into it and make their homes. Each spring sand martins fly into the bank and peck out the sand to make tunnels in which to nest. Lower down, the water vole makes its burrow and, at one place along the river, is the home of an otter.

The meandering river is the home to many plants and animals and they are all linked together in food chains. Insect larvae and snails feed on water plants. They in turn are eaten by fish, which fall prey to kingfishers and herons. The shallow bank where the water flows very slowly is a place where frogs breed. Many of their tadpoles are eaten by dragonfly nymphs. Those that survive change into frogs, which eat insects and worms along the riverbank. The leaves of the riverside trees are eaten by insects, which in turn are eaten by small birds such as the willow warbler. A line of trees with its feeding birds is also a hunting ground for a bird of prey called the sparrowhawk. It flies quickly along the riverbank, weaving in and out of the trees. Its flight sometimes frightens the birds and they take to the air. When this happens, the sparrowhawk catches one in its talons and flies away to eat its meal.

When the river engineers arrived they immediately recognised the problem.

"This river needs straightening," said one engineer. "That will make the water flow away quicker so it will not flood."

"The trees will have to come out," said another engineer. "They are slowing down the water too. In fact the whole bank needs shaping to give it a flat surface tilting at 45 degrees away from the water. This will help the water move away faster when the river rises."

"What about the river bottom? It is full of sand and grit?" asked a third engineer. "We'll have to dredge the river and remove it."

The villagers agreed to the engineers' plans and soon diggers were at work in the river and chain saws droned on the riverbanks as the trees were cut down.

Six months later a kingfisher flew along the river. It had no branches on which to land so just kept going. When the sparrowhawk visited the riverbank it just kept going too and never returned. In the water, some small plants had managed to grow on the river bottom. Snails and some insects were eating them. Occasionally a duck landed on the water and dipped its head under the water to take a leaf. A few small fish fed on the snails but there was not enough food for larger fish.

In addition to the broken food chains, the homes of the otter, vole and sand martin were gone and all had moved away to try and find a home elsewhere. In the village there was rejoicing in the winter. Not one home was flooded. River water did not even flow down the village street. As one old villager put it, "Those engineers did a right good job!"

Does every river have an otter?
No. Otters are rare although their numbers are slowly increasing in some places. Otters are nocturnal animals and hunt for fish at night. They may hunt over 19 to 25 kilometres of river so if an otter is present on a river it could be anywhere along a long stretch of riverbank. Otters have a brown coat, a small head, long body and long tail.

You can see animals with long brown bodies in the daytime on some rivers. Are they otters?
Probably not. They are almost certainly mink. The natural habit of these animals is in North America. The ones seen on riverbanks in Britain have escaped from mink farms. Minks live on their own except in the breeding season. They are active during the day and feed on fish, mice and water birds. They will even catch and eat squirrels and rabbits.

Is a water vole a water rat?
There is no such animal as a water rat. The water vole is often called the water rat because from a distance it looks like a rat. The water vole may be seen swimming across a river or along a riverbank during the day. Voles are similar to mice but have a rounded nose instead of a pointed one. They also have small rounded ears. In fact their whole body has a rounded shape a little like that of a guinea pig. A water vole is unusual because it is much larger than other kinds of vole and is a little smaller than a rat. It is easy to tell from a rat because a water vole does not have large pink ears and it does not have a long, pink, almost hairless tail.

What do water voles eat?
They eat plants that grow along the riverbank. They also store acorns and nuts. In very wet and cold weather, a water vole stays in its burrow and eats some of its store of food.

What are sand martins like and what do they eat?
Sand martins are like swallows. You can tell a sand martin from a swallow in the following way. Swallows have blue feathers on their back and are white underneath. Sand martins have brown feathers on their back and are white underneath. Sand martins feed on insects which are flying above the river. These insects have spent their early lives in the river water then changed into adults to fly through the air.

What happens to the plants and animals in the river when it is straightened and dredged?
They are washed away. The animals may find homes lower down the river where the engineers have not been at work. This is possible because in some places the river is encouraged to flood so the water can enrich the soil in the nearby fields. It is unlikely the plants would take root again and they would almost certainly die. However their seeds may land on a bank and sprout.

What happens to the birds and mammals that live on the river?
They would have to search for new homes. This is not as easy as it sounds because other river habitats will already have most of the animals that they can support. The animals may have to look for a long time to find somewhere else to live. If they do not they will certainly not breed and when they die they will leave no offspring behind. This means that the numbers in an animal species can be reduced when rivers are straightened.