SB 6.10.16
tatah suranam asurai
ranah parama-darunah
treta-mukhe narmadayam
abhavat prathame yuge
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Thereafter, at the end of Satya-yuga and the beginning of Treta-yuga, a fierce battle took place between the demigods and the demons on the bank of the Narmada.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Herein the Narmada does not mean the Narmada River in India. The five sacred rivers in India — Ganga, Yamuna, Narmada, Kaveri and Krsna — are all celestial. Like the Ganges River, the Narmada River also flows in the higher planetary systems. The battle between the demigods and the demons took place in the higher planets.
The words prathame yuge mean “in the beginning of the first millennium,” that is to say, in the beginning of the Vaivasvata manvantara. In one day of Brahma there are fourteen Manus, who each live for seventy-one millenniums. The four yugas — Satya, Treta, Dvapara and Kali — constitute one millennium. We are presently in the manvantara of Vaivasvata Manu, who is mentioned in Bhagavad-gita (imam vivasvate yogam proktavan aham avyayam/ vivasvan manave praha). We are now in the twenty-eighth millennium of Vaivasvata Manu, but this fight took place in the beginning of Vaivasvata Manu’s first millennium. One can historically calculate how long ago the battle took place. Since each millennium consists of 4,300,000 years and we are now in the twenty-eighth millennium, some 120,400,000 years have passed since the battle took place on the bank of the river Narmada.