देवेषु विश्वासटिप्पन्यः

Possibiities

1. the sarasvatI channel was a holy river to Harappans from their heydays & they transferred that sanctity to the Arya-s with whom they mixed& the Arya-s identified it with their own ancient riverine traditions.
1. The rain fed river was still active enough for the first 200-300 yrs of Arya settlement. Thereafter the Arya-s witnessed its drying up and recorded the same faithfully in the texts.