Sapta Sindhu
The Sapta Sindhu are a group of seven chief rivers of uncertain or fluctuating identification (the number seven is of greater importance than the exact members of the group, compare the Saptarishi, and also the (later) seven seas and the seven climes) of the Avesta. The Avesta's hapta həndu are preemptively equated with the Vedic Sapta Sindhavaḥ or vis-a-vis: in Vendidad 1.18 these are described to be the fifteenth of the sixteen lands created by Mazda.
Read more about this topic: Rigvedic Rivers