“Lomasa said, ‘The blessed God having heard what Bhagiratha had said, and with a view to doing what was agreeable to the residents of heaven, replied to the king, saying, ‘So let it be. O most righteous of the protectors of men, O (prince) of a powerful arm! For thy sake I shall sustain the river of the gods, when she will take her descent from the sky, she who is pure and blessed and divine, O (king) of a mighty arm!’

In plain words: Shiva, wanting to please the gods, told Bhagiratha, "So be it." He promised to hold up the pure, holy river Ganga when she came down from the sky.