James Dickey's complex novel cannot be reduced to a single theme, but a central point most readers would probably agree on is that the story is a parable of the inexplicable nature of human cruelty. The four men wish to bond with each other and commune with nature, but they are sidetracked by the unexpected danger that lies not in the river rapids but in other men and, ironically, within themselves as well.

A corollary of this theme is the dichotomy Dickey sets up between, on the one side, the sophistication of Ed, Lewis, Drew, and Bobby, who all lead comfortable lives in Atlanta, and on the other, the wild and uncontrolled rural life surrounding the river. Though Lewis thinks himself a citizen of both worlds, so to speak, the message is that this uncivilized factor even in the midst of the most advanced country in the world is a power that can humble men in their complacency and shatter their pretensions to some superior mode of existence.

When Bobby is attacked and Lewis kills the attacker, the four men are tested morally in parallel to the physical test they have willingly exposed themselves to on the raging river. The decision is made, with Drew the one holdout against it, to bury the body instead of reporting the incident to the authorities. This, Dickey seems to assert, is another instance of the same human cruelty inflicted in a different form upon Bobby.

When the men start their escape downriver, the wildness of nature seems to burst forth as a punishment for their misdeed. Drew is killed, Lewis breaks his leg against the rocks and is plunged into agony, and from this point the struggle does become a purely physical one in which the men must revert to the pre-civilized, animal-like condition of our remote ancestors.

The central theme revolves around a question posed by all of this (and for the most part left unanswered): is savagery the essence, the true condition, of human life, regardless of how often we tell ourselves that this is not so, that we are "civilized" and have overcome the baser instincts?