What are the themes in The River?

One of the themes of The River is the triumph of the human spirit. As in his earlier adventures in Hatchet, Brian is forced to dig deep into his innermost reserves to overcome extreme adversity. Only this time, he is not the only one experiencing such adversity; now, he is accompanied by Derek, a government psychologist. It was hard enough for Brian to deal with the harshness of life in the wilderness when he was on his own but now that he has someone else to take care of—someone without any experience of living out in the wild—things are even more difficult for the reluctant young adventurer.
As we learned in Hatchet, there is clearly something special about Brian that allows him to survive in such dangerous conditions. It is not just that he has developed outstanding survival skills—though they are still important—it is also that he possesses a strong spirit that remains unshakable in the face of everything that Mother Nature has to throw at him.