How does Ken Sleight believe that Everett Ruess died in Into the Wild?Ken Sleight is an expert of the Davis Gulch area

Ken Sleight believes that Everett Ruess drowned in the San Juan River.

Sleight is a professional river guide who has visited all the places Ruess visited, having lived and worked in the area for over forty years.  He has "spent as much time investigating the riddle of Everett Ruess as any other person", and is convinced the young man drowned in 1934 or early 1935.  While hiking down Grand Gulch, Sleight had discovered the name "Nemo", an alias adopted by Ruess, carved into the soft mud of an Anasazi granary.  Sleight believes that after departing Davis Gulch, Ruess hid all his gear in a cave and headed down to visit Indian friends on the Navajo Reservation.  His route would have taken him across the Colorado River, along a rugged trail across Wilson Mesa and the Clay Hills, and then down Grand Gulch to the San Juan River.  Ruess would have had to cross the river to reach the reservation, and it is here that Sleight believes he died. 

Sleight thinks that if Ruess had made it across the river and reached the reservation, he would not have been able to keep his presence a secret.  He bases his theory on Ruess's tendency to disappear for awhile, then come back to civilization, only to disappear again.  Sleight believes that since one of the last known signs of Ruess's presence appears at Grand Gulch, after which he is never heard from again, he must have met his demise somewhere there along the river (Chapter 9).