Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 36: Umā Curses the Demigods
Text 1.36.3

त्रीन्पथो हेतुना केन प्लावयेल्लोकपावनी॥

trīn patho hetunā kena plāvayel loka-pāvanī

trīn = the three; pathaḥ = paths of Bhū-loka, Bhuvar-loka and Svar-loka; hetunā = for reason; kena = what; plāvayet = did flood; loka-pāvanī = the purifier of the worlds.

For what reason did the purifier of the worlds flood through the three paths of Bhū-loka, Bhuvar-loka and Svar-loka?

This is Lord Rāma’s second question about Gaṅgā.