Asthi-visarjan [अस्थि-विसर्जन] is a very important religious ritual in the Hindu religion. Asthi means the left over bones and some ashes of a dead person collected after performing the last rites.
After the last rites are performed the remains of the dead person are collected, these are mostly tied in a peace of cloth. These ashes are then immersed in a clean normally flowing water body like a river. The practice of immersion is known as Asthi-visarjan.This practice is a must for all Hindus.
It is commonly believed that immersion of the ashes in the river Ganga is most sacred.Hence many devote Hindus do the immersion ceremony at places like Haridwar & Varanasi.
As per all Hindu religious texts when a person dies, his relation with the material body ends. The soul leaves the body and its next destination depends on the cumulative KARMA of all his previous births. The body if it has completed its process of evolution merges into the Ever-Existent or God, if not then it is prepared for the next birth. The soul is reborn again and again in different material bodies till it finally completes its evolution process.
The material body is cremated to reduce it to its basic elements, during the Cremation the gasses and other compounds present in the body are freed and they merge into the atmosphere. The body consisting of the Five Elements of Earth, Air, Water, Fire and Ether, has already merged into the Elements during the Cremation Ceremony. What is left over are some bones or ashes, these have to find a resting place hence the Asthi-visarjan Ceremony.