Ishmael,
Thanks for that.
I reckon the ashes would have been transported from Durrington Walls, along the Avon, by boat, to the Avenue, then processed along that to Stonehenge for a ceremony, on the midwinter solstice, before final internment in the ground either there (for the elite), or elsewhere for the plebs. I can't "feel" a reason for deposition in the river.
Cheers,
TE.