According to Kashi Khand, Chapter 28, Lord Shiva was describing to Goddess Parvaty the importance of River Ganga. A long time back there was a Brahmin named Vahikan. Though he had adorned the sacred thread (Yagyopaveeth), he neither observed any rituals nor followed any code of conduct prescribed for Brahmins. He never uttered any prayers. He was never interested in chanting vedic hymns and he married a girl from out of his caste.
Once there was a severe famine and left with no alternative he and his wife left their home for greener pastures. On the way they had to cross a forest, where he was attacked and killed by a tiger. A vulture saw his dead body, took a piece of his leg and flew away. In the sky, it encountered another vulture who fought with the first vulture for sharing a piece of flesh and in the scuffle one small piece of flesh and bone fell below. It so happened that coincidentally both the vultures were flying over the River Ganga and the piece of bone fell into the holy river.
After Vahikan’s death, the attendants of Yam Raj took him and produced before Yam Raj. Chitragupta listed out the evil doings by Vahikan. Kashi Khand has described in detail the misdeeds and sins committed by the dead person. Accordingly Yam Raj ordained that Vahikan should be awarded the severest of all punishments and he should go through the travails of various hells like Raurava, Anda Tamisra, Kumbi Paka, Krimi Bhojana etc.
Just when Vahikan was about to suffer the punishments, his bones fell into the River Ganga following a fight between the vultures. Several celestial damsels from Heaven (Apsaras) descended on the earth and took Vahikan, who had now assumed a divine form, to heaven. This shows the power of immersion of bones in the River Ganga.
To this day, people immerse the Asthi of their departed relatives in Holy River Ganga.