Lord Vishnu, the enjoyer of all sacrifices, appeared as Vamanadeva in the sacrificial arena of Bali Maharaja. Then He extended His left foot to the end of the universe and pierced a hole in its covering with the nail of His big toe. Through the hole, the pure water of the Causal Ocean entered this universe as the Ganges River. Having washed the lotus feet of the Lord, which are covered with reddish powder, the water of the Ganges acquired a very beautiful pink color. Every living being can immediately purify his mind of material contamination by touching the transcendental water of the Ganges, yet its waters remain ever pure. Because the Ganges directly touches the lotus feet of the Lord before descending within this universe, she is known as Vishnupadi. Later she received other names like Jahnavi and Bhagirathi. After one thousand millenniums, the water of the Ganges descended on Dhruvaloka, the topmost planet in this universe and then to the seven planets beneath Dhruvaloka. Then it is carried to the moon by innumerable celestial airplanes, and then it falls to the top of Mount Meru, which is known as Sumeru-parvata. In this way, the water of the Ganges finally reaches the lower planets and the peaks of the Himalayas, and from there it flows through Hardwar and throughout the plains of India, purifying the entire land.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Fifth Canto, Chapter 17 – Text 01 & 04