The question is, Bazz, what can the downstream states do?

Also, would the Chinese leadership be dumb enough to make nuclear armed India desparate?

The Vietnamese are already suffering as a result of Chinese tampering with the flow of the Mekong river. But Vietnam is helpless in this situation.

Another upstream state that seems to be making a water grab is Turkey.

Truly we live in interesting times.

Not much anyone can do.
Just tell the people that there is not enough water and suggest they
migrate somewhere. Trouble is they are in the main poor people and do
not have the resources to pay people smugglers.
However perhaps one or two hundred million will have enough money to do just that.