Vamana, when He took three steps, stuck His foot through the covering of this universe. Water from the Causal Ocean flowed through the hole that His foot made, and it is said that that water became the river Ganges. Therefore the Ganges, flowing from the Himalayas down to the Bay of Bengal, is accepted as the most sacred water of Visnu and is worshiped by all Hindus.

From Srila Prabhupada’s Cc Adi Introduction