Ganga Comes Down to Earth :

A legend from the Ramayana speaks of King Bhagirath who once meditated before Lord Brahma for a thousand years for the salvation of the souls of his ancestors. Pleased with his devotion Brahma granted him a wish. He requested the Lord to send the river Ganges down to earth from heaven so that she could flow over his ancestors' ashes and wash their curse away and allow them to go to heaven.
Brahma granted his wish but asked him to pray to Shiva, for he alone could support the weight of her descent. Accordingly he prayed to Shiva and he allowed the Ganges to descend on his head, and after meandering through his thick matted locks, the holy river reached the earth. This story is re-enacted by bathing the 'linga'.