According to the legend, King Sagara of the Ikshvaku dynasty ruling at Ayodhya had two queens, Keshani and Sumati, but neither had a child. Sagara performed severe austerities before his wives could produce sons. But whereas Keshani gave birth to a son called Asmajas, Sumati bore 60,000 sons.
Sagara performed the Ashwamedha sacrifice to declare his suzerainty over the neighbouring kingdoms. According to the prevalent custom, the sacrificial horse was let loose and allowed to wander into the neighboring kingdoms. If the horse was caught, a battle ensued and the outcome decided the winner. The 60,000 sons of Sagara were following the horse when they saw him enter a cavern where sage Kapila was meditating. Not seeing the horse in the cavern, they presumed that Kapila had captured it. They did not kill Kapila as he was a sage but they started disturbing his meditations. Annoyed at being disturbed, Kapila with a curse burnt the 60,000 sons of Sagara.
Since neither the horse nor his sons returned King Sagar became anxious. He sent his grandson Anshuman to search for them. Kapil Muni told Anshuman the whole story. Anshuman asked, “How can the sin of the king’s boys be destroyed?”
Anshuman tried but did not succeed in bringing down the Ganga on earth. Then his son Dilip also tried, but without success. Finally his son Bhagirath performed austerities to bring down the Ganga to this earth. Bhagiratha prayed to Brahma, the Creator. Brahma asked him to pray to Vishnu, the Preserver, to allow the heavenly Ganga, issuing from His big toe, to come down to earth. Vishnu when prayed to by Bhagiratha agreed, but asked him to request Shiva, to allow the torrential rain to fall on his head before it came to the earth as the river was very forceful and if she were allowed to come down unchecked, her fall would split the earth. Shiva agreed to take the gigantic weight of the cascading Ganga on the matted hair piled high on his head. This delayed the progress of the river which, in meandering through the labyrinth of his hair, lost its force and then gently descended to the Himalayas from where it flowed to the plains bestowing its waters on the parched earth. And that is why the anthropomorphic image of Ganga is shown in the matted hair of Shiva who is also called "Gangadhara".
It is on the tenth day of the bright half of Jyeshth month, mother Ganga came on this earth. That is why Ganga Dussehra festival is celebrated on the tenth day of Jyeshtha. The festival is also called Ganga Puja festival.
Being born in the Himalayas, Ganga is considered the elder sister of Parvati, who is also a daughter of the Himalayas.
According to the Agni Purana and Padma Purana, the Ganga descended to the earth on Ganga Dussehra day and a bath in the holy river on this day is said to purify one of all sins. To die on the banks of the Ganga is considered most auspicious. If that is not possible, then the immersion of the ashes after cremation in the river Ganga is a must, as it then releases one from the cycles of birth and re-birth.
Seven ways of worshipping the Ganga are: by calling out her name, 'Jai Ganga Mata'; having darshan of her; by toughing her waters; by worshipping and bathing; by standing in the waters of the river; and by carrying clay dug out of the river.
Ganga in her anthropomorphic form is shown as a beautiful young woman sitting on a crocodile and holding a waterpot in her hands.
The Ganges is believed to flow in the three worlds:
In heaven she is called "Mandakini", on earth as the "Ganges (or Ganga)", and in the nether region she is called "Bhagirathi". Thus the Ganga is known as “Tripathaga”, or the “Three Path River”.
Initially, river Ganga flowed in the heavens. She was brought down to earth by the severe penances of the sage Bhagiratha and that is why she is also called "Bhagirathi".
Ganga is supposed to be the daughter of the Himalayas and the goddess Mena. According to the Puranas, the heavenly Ganga flows from Vishnu's toe. Ganga sometimes assumed a human form. In one such appearance, she married King Shantanu and was the mother of Bhishma. To know more about story of Ganga and her sons, check the following post in this blog:
Whether you are in Gangotri (the place where she manifests first in this realm) or if you are at (H)Rishikesha or Haridwar, Banaras (Varanarsi), Allahabad (Prayag) or in Mayapur, West Bengal, or remembering those wonderful places, the purifying association of Mother Ganga remains the same.
In Srimad Bhagavatam, 5th Canto, 17th Chapter describes the origin of the Ganges River, how it flows and from where it flows. Read Srimad Bhagavatam 5.17.1 to 5.17.11. Check the following link to start reading from Srimad Bhagavatam 5th Canto 17th Chapter 1st Verse: