There are two main reasons behind the cause to flow the remains of the dead body in pious rivers like Ganga etc. According to *Kooram Puran, Chapter 35, Shlokas 31-34* -
*Yavadshteeni gangayaam tishthanti purushasya tu |
Taavad varsh sahasraani swargloke maheeyate ||
Teerthaanaam paramam teerth nadeenaaam parmaa nadi |
Mokshdaa sarvbhootaaanaaam mahaapaatkinaamapi ||
Sarvatra sulabhaa ganga trishu sthaaneshu durlabhaa |
Gangadwaare prayaage cha gangasagarsangme ||
Sarveshamev bhutaanaaam paapopahatchetasaam |
Gatimanveshamaanaanaam naasti gangasamaa gatih ||*
It means for as many years the remains to the body remain in Ganga till that much thousand years he is worshiped in heaven. Pious Ganga is known the main shrine in all the shrines and Superior river in all rivers. Ganga grants salvation to all living beings, so much so, it can grant salvation the most sinners. Ganga being available to all everywhere. Haridwar, Prayag and Ganga Sagar are the three main pious shrines. For those wishing supreme state and for those who have ill minds there is no other state other then Ganga.
The scientific reason behind flowing the remains after *Anteshthi* ceremony, the remains of the dead body and the ash into pious river Ganga etc., is that Ganga river flowing for hundreds of kilometers of land, irrigates and makes it fertile. The power of Ganga making the land fertile remains due to the floating remains of the dead body. In fact the bones contain Phosphorus and Phosphorus is considered to be one of the main ingredients of fertility.