Water pollution is caused due to several reasons. 

Here are the few major causes of water pollution: 

(i) Discharge of sewage and waste water:

1. Sewage, garbage and liquid waste of households, agricultural runoff and effluents from factories are discharged into lakes and rivers.

2. These wastes contain harmful chemicals and toxins which make the water poisonous for aquatic animals and plants.

(ii) Dumping of solid wastes: The dumping of solid wastes and litters in water bodies cause huge problems.

(iii) Discharge of industrial wastes: Industrial waste contains pollutants like asbestos, lead, mercury, grease oil and petrochemicals, which are extremely harmful to both people and environment.

(iv) Oil Spill: Sea water gets polluted due to oil spilled from ships and tankers while travelling. The spilled oil does not dissolve in water and forms a thick sludge polluting the water.

(v) Acid rain:

1. Acid rain is pollution of water caused by air pollution. 

2. When the acidic particles caused by air pollution in the atmosphere mix with water vapor, it results in acid rain.

(vi) Global warming: Due to global warming, there is an increase in water temperature as a result aquatic plants and animals are affected.

(vii) Eutrophication:

1. Eutrophication is an increased level of nutrients in water bodies. 

2. This results in bloom of algae in water.

3. It also depletes the oxygen in water which negatively affects fish and other aquatic animal population.