Many towns and factories are built near rivers so that they can use water from the river. But sometimes the water is not cleaned before it is put back into the river. This dirtying of water (also air and soil) is called pollution.
Pollution can be in a variety of forms:
Causes of pollution
Farmers put fertilizers and pesticides on their crops so that their crops grow better. But these fertilizers and pesticides can be washed through the soil by rain, to end up in rivers. Fertilizers and pesticides are harmful because they cause algae to grow. The algae then destroy water plants.
Factories use water from rivers to power machinery or to cool down machinery. Dirty water containing chemicals is put back in the river. Water used for cooling is warmer than the river itself, and this is also a form of pollution.
People are sometimes careless and throw rubbish such as bottles and crisp packets directly into rivers.
Effects of pollution
Efforts are being made to clean dirty rivers. Water companies like Southern Water are spending a lot of money to clean up dirty rivers. There are fines for companies and people who deliberately throw rubbish into the river.