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.