Question

who inhabited the upper plain of river ganga in later vedic period

Answer 1

The Aryans.

After 1000 BCE, the use of iron axes and ploughs became widespread and the jungles could be cleared with ease. This enabled the Vedic Aryans to extent their settlements into the western area of the Ganga-Yamuna Doab (i.e. upper plain of the River Ganga).