There are numerous stories in Hindu tradition regarding the origin of Ganga. One such story states that the Origin of Ganga happened during the Vamana Avatar – Vishnu’s incarnation as the dwarf Vaman.
Legend has it that King Bali was a performing a Yajna to obtain the throne of Indra. All Devas approached Vishnu to help them from the expected rule of King Bali. Vishnu promised to help them and appeared in the form of Vamana – a dwarf.
On seeing Vamana in the Yajna, King Bali received Him and prayed to him to take some gift from him. Bali told Vamana that whatever He demands, he will give.
Vamana demanded just three steps of land and He said He would like to measure it all by himself.
King Bali was surprised at this simple request and agreed to it.
Soon, the short statured Vamana grew in size – so huge that it was impossible to describe in words.
With his first step, Vamana covered all of Earth. His second step covered all of the Heaven. There was no place for the third step and King Bali offered his head. Vamana sent Bali to Patalaloka or Underworld.
It is said that when the foot of Vamana touched heaven, Brahma washed it with holy water – this holy water took the form of Ganga and travelled from heaven to earth.
Note – the most famous origin of Ganga is the one associated with King Bhagiratha brining Ganga down to wash the sins of his ancestors.