DNA analysis indicates that dolphins endemic to the Indus and Ganges rivers are closely related. Since the river systems aren’t connected, and the dolphins are much closer to each other than those found in the Indian Ocean, it is very likely that the river systems were connected at some point in the past.
In the book ‘The Incredible history of India’s Geography’ the author, Sanjay Sanyal argues that this ‘connection’ could have been the mythical Saraswathi. There seems to be a lot of scientific evidence that points to this, but at this point I’d say it’s interesting, but far from conclusive.
I can’t help but think of this separation of the river systems as an apt metaphor for the separation of the present day nations of India and Pakistan (and Bangladesh a few decades later). Just like the geo-physical event of the drying of the Thar desert (might have) lead to the separation of the two river systems (the the dolphin populations), the geo-political event of partition has lead to the separation of the two nations and it’s human population.
Ironically, the Ganges dolphin is the India’s national aquatic animal while the Indus dolphin is Pakistan’s national mammal.