In the three worlds there exists none who is able to sustain the same, excepting Siva, the most praiseworthy of gods, the great Lord with the throat of sable blue. O (prince) of a powerful arm! Obtain the favour, by practising austerities, of that same Siva-giver of boons. That same god will sustain my descent upon his head. Thy desire he will fulfill, the desire, namely, to be of service to thy fathers, O king!’

In plain words: Ganga said only Shiva, the god with the blue throat, could bear her fall. She told the king to pray hard to win Shiva's favour, so Shiva would catch her on his head and the king could help his ancestors.