How did the Indus valley civilization end?

there are many theories on this like aryan invasion,drought,famine e.t.c but there are no proofs for these,they are just rumors.but according to UN's statement a powerful flood attacked indus valley civilisation bringing end to the world's first and most advanced civilisation.and i too have similar ideas as this is a place of powerful rivers like indus,ravi,saraswati,chenab,satluj,yamuna and according to some theories even ganges was flowing here.