Ganga is related to Shiva as Yamuna to Krishna.
Ganga’s mount is the crocodile, Makara.
Because she was brought to earth by Bhagiratha she is also called Bhagirathi. In Devprayag the rivers Alaknanda and Bhagirathi become Ganga.
Ganga’s creation is related to Vishnu’s Vamana Avatar. When he stretched his foot towards heaven, Brahma washed it and caught the water in his water fessel.
Why Ganga had to come down to the earth is told in two myths.
The story of Ganga
The story of Ganga and Shantanu