The surface area of water at basin scale can only be measured through satellite data. You can compare two methods namely visual interpretation based area and digital classification based area.

Reservoir spread areas may be available in the records of the concerned Government Departments, but I very much doubt about the water spread areas along the river courses, that too of the dimension of Ganga River Basin.

I think ..Dharmendra sir is right..you can calculate it with RS imagery and I think such kind of information is rare which may be find from Government organization.

You can validate the estimated flooded area for specific events. Here you have one example of runoff global data: http://flood.umd.edu/