By performing very severe austerities, King Bhagiratha received the benediction from mother Ganges that she would descend to the earth planet. But she was afraid that her forceful waters would pierce the surface of the earth and continue down to the lower planetary system. 

King Bhagiratha reassured her: "Like a cloth woven of threads extending for its length and breadth, this entire universe, in all its latitude and longitude, is situated under different potencies of the Supreme Personality of Godhead. Lord Shiva is the incarnation of the Lord, and thus he represents the Supersoul in the embodied soul. He can sustain your forceful waves on his head." 

After saying this, King Bhagiratha performed further austerities and very quickly satisfied Lord Shiva. Thus, when the king approached Lord Shiva and requested him to sustain the forceful waves of the Ganges, Lord Shiva accepted the proposal, saying, "Let it be so." Then, with great attention, Shiva sustained on his head the torrent of Ganges water, which is purifying, having emanated from the toes of Lord Vishnu.