The story of Ganga coming to earth did not start just like that. A whole generation is there, to tell you the story. There was a king called Sagar, he was performing the ‘Ashwamedh Yagna’ to be more powerful. The king of all gods, Indra got scared and stole the horse of the Yagna. He left the horse in the Ashram of Muni Kapil. King sent his ten thousand sons to find the horse, they reached Kapil and blamed him for stealing the horse. Muni Kapil got angry and turned all of them into ashes.
After that many generations passed, but these 60,000 princes did not get salvation. In the 7th generation a king called Bhagirath was born and he promised his ancestors that they will get salvation. He did Tapasya for years and then pleased God Brahma. Brahma told him that his ancestors will get salvation only if river Ganga will come on earth. But the problem occured when Bhagirath came to know that only one drop of Ganga can drown the whole earth.
Then Bhagirath prayed to lord Shiva and requested him to take Ganga in his hair. Shiva agreed, and Brahma dropped a single drop from his pan. Shiva took it in his Jatas. Then he left a strand on the earth and thats how Ganga came to the earth and 60,000 princes got salvation.