The story of the holy river Ganga (Ganges)

A long time ago in the heavens, lived a beautiful Goddess called Ganga. She was very playful, very powerful and very pure. Everything she touched became pure like her. A king on earth wanted to bring her down to earth so that she could purify everyone. Finally, Ganga agreed to come down. but she came down from heaven with such force that she would have shattered the earth. Lord Shiva had to step in and catch her in his hair to slow her flow. Hindus say that everyone who takes a dip in the river Ganga becomes purified by her holy touch.

Reference: Primary Hinduism by Seeta Lakhani (Ch.5, 'Worship And Celebration', p.65)