I should have elaborated further.
The Sapta Sindhava ("Seven Rivers") is a region described in the Rigveda and Zend Avesta. It comprises of the Sarasvati, the Indus and it's five tributaries.
To the early Indo-Aryans, the 'Sarasvati' was most likely the Helmand river. The Avestan term for the Helmand river was Haraxvaiti which is a cognate of the Sanskrit Sarasvati. Both the Avesta and Rigveda describe Haraxvaiti/Sarasvati with astonishing similarities and the Rigvedic description of the Sarasvati matches the geography of the Helmand river.
The Sarasvati described in the post-Rigvedic texts is most likely a different river compared to the Sarasvati that was known to the early Indo-Aryans.
Rajesh Kocchar has extensively written about this I believe, if you'd be interested to look more into it.
Well, I have a reconstruction, if you would care to read it.
You will recall that some authors, I think Parpola was among them, had speculated that the Gandhara Grave Culture was constituted of two sets of remains, remains of the original inhabitants of the IVC, and remains of the steppe migrants. The speculation was that there were two waves of migration, one, around 1900 BCE, of people of that genetic profile, but without the connections to the Rg Veda and other parts of the Vedic theogony.
This was with very little concrete evidence.
It is also clear from the Vedic references that there were two distinct references to the Saraswati. One - the earlier - spoke of it as a foaming river. The other, the later, spoke of it as a river that disappeared into the desert. From the reconstructions of the writing of the Vedas, it is apparent that there was a significant gap between the writing of the earlier cycles and the later cycles.
We also know that the Persian equivalent, as you have pointed out, in your latest note, for instance, was Haraothi. We know also that such a river exists, not just in ancient times but today, and that this river flows into the Helmand.
Finally, we have the incident that was reported as the Battle of the Ten Kings. In this, the Bharata tribe, ancestor to both the later Kurus and their branch, the Pandavas, and located in the Punjab-Haryana area, fought and defeated a confederation of many kings from southern and western parts. The scant references seem to indicate that there were older and more settled, and more powerful tribes among the opponents, and that the Bharata tribe was not dominant before this battle, but was dominant after their victory in this battle.
This gives rise to a lot of possibilities.
What if the earlier branch of the migrants had not merely penetrated to Swat, but had also gone straight on, and reached the Afghan Haraothi flowing into the Helmand? They would have had easy access to the Gomal Pass and found themselves among the earliest settlers, and presumably the better established ones, and have been settled along the Indus up to the five rivers region.
What if the Bharata tribe and many others had come in a later wave and found, on crossing the Khaibar, that the way to expand, along the Indus, was blocked, and therefore spread out to the east?
That would explain that the earlier reference to the Saraswati was to the foaming river that the southern settlers had actually seen (and named) flowing into the Helmand, and that the later reference was to the river that flowed into the desert. This later reference is a very weak one, and the river that it is thought it might refer to, the Ghaggra Hakra, is also a v ery weak one. It was established geologically that it was weak even in those early days.