Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 43: Gaṅgā Descends to the Earth
Text 1.43.8

तदा सातिमहद्रूपं कृत्वा वेगं च दुःसहम्।
आकाशादपतद्राम शिवे शिवशिरस्युत॥

acintayac ca devī gaṅgā parama-durdharā
viśāmy
ahaṁ hi pātālaṁ srotasā gṛhya śaṅkaram

acintayat ca = thought thus; = the; devī = goddess; gaṅgā = Gaṅgā; parama-durdharā = supremely unrestrainable; viśāmi = will enter; aham = I; hi = certainly; pātālam = Pātāla; srotasā = with a stream; gṛhya = holding on; śaṅkaram = to Lord Śaṅkara.

The supremely unrestrainable goddess Gaṅgā thought thus: “Holding on to Lord Śaṅkara with a stream, I will certainly enter Pātāla.”

These are the reasons for Gaṅgā’s assuming a great form and an intolerable speed.