The main causes for water pollution are :
Disposal of waste:
This waste includes sewage, garbage and liquid waste from factories and homes. Wastages from chemical factories contain many toxic or poisonous chemicals. These are discharged into rivers. The river-water becomes poisonous for fish and other aquatic animals and plants.
Chemical fertilizers and pesticides:
Farmers use fertilizers for growth, and pesticides to kill insects that damage crops. Fertilizers and pesticides which are good for plants, may be harmful to human beings and animals. Many of these chemicals get dissolved in rainwater which flows into ponds, canals, rivers and seas.
Disposal of human excreta and animal dung:
The rain washes away human excreta and animal dung into rivers, ponds and lakes. Drinking this polluted water may cause several water-borne diseases such as diarrhea, dysentery, jaundice, typhoid and cholera. Polluted river-water can cause diseases to the whole population.