Hindus cremate the dead bodies by burning them. The remnant of the dead bodies is in the form of ashes, which Hindus immerse in holy rivers like the Ganga.The process is called as Asthi Visarjan. The meaning of Asthi is the left over bone or some collected ashes of dead people and Visarjan is to immerse. If it is not possible to immerse them immediately, they are put in a crematorium and immersed as soon as possible.
There are various theories behind the significance of dipping ashes in the holy rivers. One of the oldest Sanskrit literature called Shankhsmriti says,
As long as the ashes remain in Ganga, the person remains to be happy in worthy place for thousand years.
It is further elaborated in Kurmpuran,
The number of years the ashes remain in Ganga; the departed soul enjoys remaining in heaven multiple times to thousand years. Of all the rivers and all the pilgrimages, river Ganga is considered the holiest one. It grants salvation to all, including those who have committed gross sins. Although, accessible everywhere to a common man, Ganga is unique at Haridwar, Prayag, and Gangasagar. Those who desire salvation, including sinners, there is no better place than the Ganga.
Story of Ganga
There was a powerful king named Sagar. He had 60,000 sons. One day he wishes to perform 100 Ashwamedha Yajna. Ashwamedha is a practice where a horse is released to all places wandering wherever it wishes and defeating anyone who catches it. If the horse is returned to its original place without being captured then it is said that one Ashwamedha Yajna is complete. It is also believed that if 100 Ashwamedha is completed then you can sit in the throne of heaven, which is on hold by Indra.
King Sagar performs Aswamedha as he hoped and 99 Ashwamedha is completed without the horse being captured. On the 100th Ashwamedha, Indra, king of deities stole the horse and tied up in the ashram of sage Kapil. When King Sagar found that his horse had been stolen, he ordered his 60,000 sons to bring back the horse.
All the sons, in the order of their father, went searching for the horse. They searched all over and found the horse tied in sage Kapil’s ashram. When they saw sage Kapil, they all ran to kill him. But the sage at his one glance burnt all of them to ashes.
A few days later, Anshuman, grandson of King Sagar asked sage Kapil for forgiveness to what his elders had done. He wanted salvation for his elders. Sage Kapil advised that they could only get salvation if Ganga is brought to the earth and wash away their ashes. This could only be accomplished if he performs vigorous meditation and fasting for several years to please Lord Brahma, God of creation.
After the advice from the sage, Anshuman began to worship Lord Brahma by fasting and meditating with full concentration. But he failed please Lord Brahma in his lifetime. So his son Dilip continued his endeavor. Nothing could happen in Dilip’s time also. Then finally Dilip’s son Bhagiratha meditated vigorously. Lord Brahma appeared before him and asked him what he wanted. He said he wanted Ganga to flow on earth so that his ancestors can get salvation and abode heaven peacefully. Lord Brahma advised him to pray to Lord Shiva for the reason and his wish would be fulfilled.
So, Baghiratha started worshipping Lord Shiva. Shiva became pleased with Bhagiratha’s devotion and agreed to help him in his endeavor. Lord Brahma orders Ganga to flow on the earth. Ganga with her proudness thought of floating the earth with her tremendous force. Then Lord Shiva holds Ganga in his matted hair. Ganga’s ego relinquished. Then Lord Shiva released Ganga from his hair and she flowed smoothly on earth and washes away the ashes of Baghiratha’s ancestors. They all got salvation after this course. Since then it is believed that immersing ashes into Ganga will provide salvation and happiness to the soul of dead people.
Scientific Reasoning
There is another belief on immersing ashes of dead ones to the river that relates to science. The water from the Ganga and the other holy rivers is used extensively to irrigate large tracts of land along the river. Since the bones are rich in phosphates their ashes increase the phosphate level in the water. Phosphates are essential ingredients for irrigating lands and growing grains. Ultimately, the elements of the body return to nature after death is achieved.