“Lomasa said, ‘Ganga, the goddess saluted by the world, having heard these words of the king, was well pleased, and spake to Bhagiratha the following words: ‘O great king! I am prepared to do what thou dost ask me; there is no doubt therein. But when I shall descend from the sky to the earth, the force of my fall will be difficult to sustain. O protector of men!

In plain words: Ganga was pleased and agreed to help Bhagiratha. But she warned that when she fell from the sky to the earth, the force of her fall would be very hard to bear.