As the Geological process began 75 million years ago and it took 50 million years for the Drift,it is logical that Ganga was formed around 25 million years ago.
I posted a couple of articles on Srardha, the Ceremonies to be performed for the Ancestors. Note that the Manvatantaras and Kali Yuga are accommodated, The Sraradhas are Ninety Six in Number,Shannavathi’ in a Year. Lord Govinda is the Ruling deity after Death' for Apara Karya, One would find Krishna being offered Srardha the Most in a year. The List.