The Ganga, living at the feet of Vishnu in heaven, is also the symbol of spiritual Gyan. To take the shock of spiritual gyan is not easy, so too Shiva is the Adi guru who takes this gyan and then trickles it for us.

Most of , you, no doubt, have heard the story of how rishi Bhagirath prayed for the Ganga to come to earth, in order to salvage his forefathers who were dead and gone. But the mighty Ganga was so ferocious in her torrential outpour that earth could not withstand her landing. Thus Bhagirath prayed to Shiva to land her in his matty hair as a buffer.Which Shiva did and then he trickled her down in good measure.But also, there is a spiritual angle to this:
The Ganga, living at the feet of Vishnu in heaven, is also the symbol of spiritual Gyan. To take the shock of spiritual gyan is not easy, so too Shiva is the Adi guru who takes this gyan and then trickles it for us. 

SHIVA'S WAY
Meditate, Receive the Gyan Ganga upon your head. Stabilize the impact. Then, trickle it down gently for the others, so that they too may quench their thirst. If you have it, give it freely.
“Simply said, but not so simple to implement. There is a very thin line between intelligence and Insanity. Consciousness means a very high intelligence, hitherto, unknown, to the new seeker. So, it has to be given in doses. So it is not to be given so freely that it leads to insanity, thus we say trickle it."