Bhagiratha performed immense tapas for years to bring mother Ganga down to earth. When he received his boon the issue was that the world would have moved from drought to deluge. The munificent Mahadeva held out his locks so that, mother Ganga would be controlled and flow restrained on to earth. One hand is holding his locks up so that ganga can descend through his locks. Ganga(holy river) dara(carrier of) Murthy(Lord).