First, I want to extend my condolences to those affected by this tragic event.
restrac2000, I did see the NPS missing person's flier for Joshua Tourjee two weeks ago. It said that he was last seen on a swan pool toy by the river at his camp site. This is a very peculiar incident. Based on the new information published in the link you shared he was last seen by the river and went missing and unfortunately his body was found 9 days later down river by NPS.
It is likely a precise cause of death will never be known.