Question

Why we should not throw polythene bag on rive r bodies

Answer 1

It is because if we do so then river water will get polluted. Polythene is inbiodegradable.
So it takes millions of yrs for it to decay.

Also if we drink polluted water we can suffer from dangerous water-borne diseases

Answer 2

we should not throw polybags on River bodies to prevent dam from any sort of disturbance.
polythene bags flow along with water and combine to create a mess of polybags which stops water to pass through dam.

This is not the end but polybags also creates water pollution. They pollutes water highly.