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.
Maghi, is month during which the famous Kumbh Mela and the annual Magh Mela takes place at Sangam in Prayag or Allahabad.
Vishwamitra performed the ritual bath during the Kumbh Mela on a Magh Purnima in 2382 BCE. Magh Purnima is an important bathing date during the Kumbh Mela and the Magh Mela.
Lord Shiva burnt the Tripura He had the Ashes that a portion of it fell in Kailash, and some in Amarkantak,Madhya Pradesh,.
This is also the place the Narmada River originates.
The presiding Deity of River Ganges visits this place, as the Legends would have it.
He was advised that he could not perform this as his ancestors, sons of Sagara were cursed by Sage Kapila(Founder of Nyaya System of Indian Philosophy considered an Incarnation of Lord Vishnu) and were turned into ashes.
They could be absolved of their sins and their State if Bhagiratha could somehow get the River Ganges down to Earth from the Heavens.
Ganges is believed to be at the feet of Lord Vishnu and the auspicious time if dribbles out of Vishnu’s feet is called Vishnupathi, equivalent to Pradosha of Lord Siva(read my post on Vishnupathi Punya Kaala).
On the advice of his Preceptors, Bhagiratha, started a Penance for Lord Siva.
His Penance was so powerful that Indra, fearing for his Kingdom disturbed it quite a few times,
For more on this please refer Vishnu Purana