While no one can be sure when the concept originated, Indian mythology equates the river Ganges with the Milky Way. According to Vedic lore, Ganga (Ganges) resided in the sky (Milky Way) and was called 'Akash Ganga,' or Sky-River. Here's the story in brief.
When the king Bhagirath performed a sacrifice to revive his dead sons by washing away their sins, Brahma asked Ganga to come down from the sky to the earth, to wash away the sins of humanity. The deluge of the great river falling from the sky would have destroyed the land, so Shiva stood on Mt. Meru in the Himalayas and allowed Ganga to fall from the sky onto the matted locks of his hair. He then slowly let her out and she flowed out from the Himalayas to the sea. King Bhagirath's sons were brought back to life as the Ganga flowed over their remains.
This is partially why the Hindus believe that bathing in the Ganga is a purifying ritual, and tens of millions make the pilgrimage every 12 years when the astronomical symbology is right: the river is aligned with the Milky way, and certain stars appear in the sky - the festival is called 'Kumbh Mela.'
The dates of the Kumbh Mela are astronomically determined, based upon precise calculations of the positions of the Sun, the Moon and Jupiter. At Prayag, the Maha Kumbh Mela is held in the month of Magha (January/February in the Gregorian calendar). The highest spiritual merit is attached to bathing on the new moon day, Amavasya, when Jupiter is in Aquarius (known in India as the Kumbh (the pot, or ‘womb’)) and both the Sun and Moon are in Capricorn. At Haridwar, the Kumbh Mela is held in the months of Phalgun and Chaitra (February/March/April), when the Sun passes to Aries, the Moon is in Sagittarius and Jupiter is in Aquarius.
In addition, Kumbh Mela is intrinsically tied to a Precession story. When the Gods and Demons were churning the 'Milky Ocean,' a few drops of the Nectar of Immortality they were trying to raise fell at these spots where the festival is held.
But while myths and symbology are important, here is where the real mystery lies: Ganges water seems to contain mysterious antibacterial and antimicrobial properties that scientists cannot explain. These myths about 'purification' and 'cleansing' where the Ganges was revered as a deity by people seem to have roots in facts that cannot yet be explained.
Strange Properties of Ganges Water
Clear spots on the petri dish mark where phages have destroyed bacteria in a sample of water taken from the Ganges River
1] Ganges water does not putrefy, even after long periods of storage. Other river water begins to putrefy when lack of oxygen promotes the growth of anaerobic bacteria, which produce the tell-tale smell of stale water.
2] The British physician, C.E. Nelson, observed that Ganga water taken from the Hooghly - one of its dirtiest mouths - by ships returning to England remained fresh throughout the voyage. This is the reason that the East India Company ships only used Ganges water for drinking on the long, 3-month voyage back to England.
3] In 1896, the British physician E. Hanbury Hankin reported in the French journal Annales de l'Institut Pasteur that cholera microbes died within three hours in Ganga water, but continued to thrive in distilled water even after 48 hours.
4] A French scientist, Monsieur Herelle, was amazed to find "that only a few feet below the bodies of persons floating in the Ganga who had died of dysentery and cholera, where one would expect millions of germs, there were no germs at all.
5] More recently, D.S. Bhargava, an Indian environmental engineer measured the Ganges' remarkable self-cleansing properties:
"Bhargava's calculations, taken from an exhaustive three-year study of the Ganga, show that it is able to reduce BOD [biochemical oxygen demand] levels much faster than in other rivers."
Quantitatively, the Ganges seems to clean up suspended wastes 15 to 20 times faster than other rivers.
(Kalshian, Rakesh; "Ganges Has Magical Cleaning Properties," Geographic, 66:5, April 1994.)
Along the Ganges people regularly draw water from the river for personal use. They keep it in big clay pots, and drink it only after 5 days, when any bacteria mysteriously disappears as described above, as compared to other water that rots if just stored. Ganges water stays sweet and fresh.
National Geographic and NPR did a study few years ago, and here is a link to their article:
"Hollick speaks with DS Bhargava, a retired professor of hydrology, who has spent a lifetime performing experiments up and down Ganges in the plains of India. In most rivers, Bhargava says, organic material usually exhausts a river's available oxygen and starts putrefying. But in the Ganges, an unknown substance, or "X factor" that Indians refer to as a "disinfectant," acts on organic materials and bacteria and kills them. Bhargava says that the Ganges' self-purifying quality leads to oxygen levels 25 times higher than any other river in the world."
"O' the good Lord some giftie gie us,
To see oursel'es, as others see us..."
~ Robert Burns