(i) undesirable substances like fertilizers and pesticides or any poisonous substances.

(ii) sewage directly entering a water body.

(iii) hot water from the power plant that increases the temperature and reduces the dissolved oxygen in water thus killing the aquatic organisms.

(iv) industrial effluents or radioactive substances in water body.

We can take following measures to check water pollution

(i) The sewer lines should not be directly connected to the water body.

(ii) We should not throw our garbages or domestic waste in the water body.

(iii) Prevent dumping of toxic compounds in the water bodies.

(iv) Washing of clothes should be avoided near water bodies as it adds lot of detergents to it.

(v) Plant trees near the banks of the river to check soil erosion otherwise erosion leads to siltation of water body.