In Australia, barramundi get well up rivers and when they partly dry up after the wet season leaving behind a serise of freshwater pools or "billabongs", the barramundi do very well.
Barra are actually stocked in a lot of reseviours and impoundments in tropical Aurstralia and they grow to huge sizes in totally fresh water.
I used to corespond with a guy from Queensland who fished a reseviour where the barra averaged 20 - 25kg's !
Those fish were stocked as fingerlings but I've read that barra, MJ's and several other estuary species can be successfully adapted from salt to fresh water in as little as 10 hours by gradually decreasing the salinity.
The only major difference between salt and freshwater barra is the colour.
Saltwater fish are bright silver with yellow tails whereas freshwater fish are usually much darker.
Predictably, saltwater barra are said to be better eating though.
Not sure how far up the Ganges and other Indian rivers they penetrate though.
Edit: Oops ! Sorry Kingfish, I just noticed your location. I rather suspect I've just tols you what you already know.