How does Krakauer know Chris/Alex intended to leave the wilderness?

Krakauer’s story of his own personal experiences makes it clear that McCandless was almost surely not suicidal. Although he admits, in his last postcard to Westerberg, that he is aware that he might never make it out of the wilderness alive, he believes in his ability to survive, and he is too young to truly be able to imagine death, especially because he has managed to survive all of his other dangerous adventures.

In Chapter 16, we learn that Chris had recorded his "to do" list for leaving the wilderness. Unfortunately, he was stopped because the river was impassable.