where did the economic power of the Ganges River come from?

Answer 1

The BBC Hindi Service's Shiv Kant travelled down the Ganges from its source to the delta.
No river in the world plays a more important economic, social and cultural role in the lives of more people than the Ganges.

Emerging from the central Himalayas, the river flows through the north Indian planes, providing water and drainage for over 350 million people.

It is a meeting point for both the rich and poor, who believe it is a divine route to heaven. Ironically, this divine status may be threatening the river's very existence.

For more info go to:http://news.bbc.co.uk/2/hi/south_asia/683566.stm

Answer 2

Did I just guess on another one from you about the coasts?

In any guess, my guess would be the same here: cheap transportation via water.