Not sure how this is a problem. The Ganges is currently way below normal flow, with some stretches completely dry, due to volume being siphoned off for irrigation and other use. Presumably it wouldn’t just be the Ganges that gets more water, either. The other big rivers would also increase, and while there would likely be some challenges during the Monsoon, the increased flow is likely to be more of a benefit than a hazard. I would imagine that, in the long run, since water is already increasingly scarce in India, they’d find some way to capture some of that increased flow for use later, such as creating large reservoirs or artificial catch basins wherever possible.
The poop scientist in me thinks that with the enormous fecal load, that yummy water may be a big boon to any crops that receive it.
Brushing your teeth though? Not so much.