Lord Gangadhara is the other name of Lord Shiva, acquired because he wore the sacred River Ganga on his head. The term Gangadhara means the one who is bearing Ganga. Sacred river ganga forms one of the attributes of Lord Shiva’s possessions in the top most part of his head, the other being the Moon God, the third eye and the horizontal strokes of Vibhoodi or Bhasmam.
Though not married to Lord Shiva, river goddess Ganga is associated with Lord Shiva in all the epics and given higher significance when compared to other river Goddesses, just for the reason that she is located above the head of Lord Shiva.
The origin of Ganga occupying the region above the head of Lord Shiva dates back to the age much before Ramayana. King Bhageeratha, forefather of Lord Rama and one of the successful kings of Surya Vansha, pledged for the release of his ancestors from a curse. Following the word of God Indra that the ashes of his ancestors have to be immersed in the river Ganga, which is located in the heaven.
Bhageeratha pleaded Lord Shiva to bring down the river Ganga, as he is the only one who can withstand the pressure exerted by Ganga falling from the Heavens.
River Ganga should start from the Tip of Lord Vishnu’s toe, travel all across the heavens and every corner of the Universe, should reach finally onto the head of Lord Shiva. Lord Shiva has held his long matted hair in such a way that the mighty flow of Ganga has to confine itself in a region on his head.