As we all know, India is a land of diverse landforms.
So, when the rivers flow down the lofty mountains, they deposit a lot of silt and mud along it's course.
Thus, in India plains are found along some major river sources:
Ganga Brahmaputra plain is the largest plain in the world that supports human habitat, due to the rivers Ganga and Brahmaputra.
Ganga is that's why called even the soul of our country.