The land where the bulk of the Rig Veda was composed is called Saptasindhu or Saptasaindhava. Avestan sources refer to the hapta-hindu. Is the term used after Rig Veda and is there a unique list of the seven rivers of the saptasindhu?
The phrase seven rivers was used many times in Rig Veda. For example:
Rig Veda VII.67.8
With one, the same, intention, ye swift movers, o’er the Seven Rivers hath your chariot travelled. Yoked by the Gods, your strong steeds never weary while speeding forward at the pole they bear you.
Rig Veda I.35.8
The earth's eight points his brightness hath illumined, three desert regions and the Seven Rivers. God Savitar the gold-eyed hath come hither, giving choice treasures unto him who worships.
However, in Rig Veda X.64.8 & 9, apart from mentioning 7 rivers, names of 3 rivers, viz., Sindhu, Sarasvatī, and Sarayu, were mentioned.
The thrice-seven wandering Rivers, yea, the mighty floods, the forest trees, the mountains, Agni to our aid, Kṛśānu, Tisya, archers to our gathering-place, and Rudra strong amid the Rudras we invoke. 9 Let the great Streams come hither with their mighty help, Sindhu, Sarasvatī, and Sarayu with waves. Ye Goddess Floods, ye Mothers, animating all, promise us water rich in fatness and in balm.
It is quite peculiar that Ganga, Yamuna were not mentioned, but Sarayu was mentioned. Was that Sarayu the same one that now flowing adjacent to Ayodhya or something different river is not forthcoming.
It might be, perhaps, due to the fact that at the time of composing these mantras, the sages either not aware of Ganga flowing in the East or the Ganga, which was described to be emanated from Shiva's head, might not have existed.