Srimad Bhagavatam Canto 08, Chapter 21, Text 04

SB 8.21.4

dhatuh kamandalu-jalam tad urukramasya
 padavanejana-pavitrataya narendra
svardhuny abhun nabhasi sa patati nimarsti
 loka-trayam bhagavato visadeva kirtih
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
O King, the water from Lord Brahma’s kamandalu washed the lotus feet of Lord Vamanadeva, who is known as Urukrama, the wonderful actor. Thus that water became so pure that it was transformed into the water of the Ganges, which went flowing down from the sky, purifying the three worlds like the pure fame of the Supreme Personality of Godhead.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Here we understand that the Ganges began when the water from Lord Brahma’s kamandalu washed the lotus feet of Lord Vamanadeva. But in the Fifth Canto it is stated that the Ganges began when Vamanadeva’s left foot pierced the covering of the universe so that the transcendental water of the Causal Ocean leaked through. And elsewhere it is also stated that Lord Narayana appeared as the water of the Ganges. The water of the Ganges, therefore, is a combination of three transcendental waters, and thus the Ganges is able to purify the three worlds. This is the description given by Srila Visvanatha Cakravarti Thakura.
Srimad Bhagavatam Canto 08, Chapter 21, Text 02-03
Srimad Bhagavatam Canto 08, Chapter 21, Text 05