Very interesting photo-essay here about the ghats at Varanasi. Good text, too.
Sometimes poorer people cannot afford enough wood to completely burn a body. In this case charred body parts are simply flung into the river with the ashes. Certain people, such as small children, pregnant women and holy men, are not cremated at all, but instead simply have their bodies weighted down with stones and are dropped into the Ganges. Not too pleasant for the many bathers around the ghats.
As a solution to the problem of human remains clogging up the Ganges, snapping turtles were bred and released into the river specifically to eat the corpses and bones. A good idea, maybe, but since bodies and body parts are still seen floating around the river today, perhaps not as effective as originally hoped.