why is the coliform bacteria in water used to indicate the level of pollution in a river?

The presence of coliform in water indicates the presence of disease causing microorganisms. 
Idk if this answers you question, well, good luck! 

your*

As the coliform bacteria level indicates that the water has been used as a source for removing human waste which indicates that the water has become contaminated

Its presence in water indicates contamination by disease causing microorganisms

Coliform bacteria is present in human intestine ,if their presence is found in water then it means it is contaminated by human dead bodies due to which water cannot be consumed n can cause disease if consumed .

The presence of coliform bacteria indicates that water is contaminated by disease causing microorganisms .

coliform bacteria is a kind of bacteria present in human intestine. It is not harmful but its presence indicates that the river might also bear the other harmful bacteria of our intestines.So it is used to indicate the level of pollution of a river inspite of it being a harmless bacteria .

hope this helps. xD