In Hindu mythology, the 47th chapter of the Preta Khanda in the Garuda Purana talks about Vaitarni River which must be crossed by the souls of the dead; it is incumbent on all mortals to witness the mighty river after death. People who had donated gifts in their lifetime can cross the river easily, others simply sink into it. Those who donate a cow to a Brahmin never fall into the Vaitarni River, clinging to the tail of the cow; the helpless soul will be able to cross the river. The Gau is considered very sacred and can lead a person to heaven or swarga.
After the death of a person the family will perform the gau daan to enable the soul of the deceased to cross the Vaitarni River and attain Moksha.