The allegory—river journey as journey into darkness of the human heart—was ripped directly out of Joseph Conrad's classic novel, Heart of Darkness, which provided the inspiration for Apocalypse Now. In Conrad's book, a sailor, Marlow, describes the journey he took up the Congo River in Africa to retrieve a Belgian businessman named Kurtz.

Marlow discovers that the Belgian government has transformed the Congo into an imperialist nightmare, subjugating and killing the local Africans while extracting all the country's resources. When he finally reaches Kurtz, he discovers that Kurtz exemplifies this madness. Even though he talks about his high ideals, he's turned himself into a ruthless tribal leader, lording it over the Africans who follow him. Taking the dying Kurtz back down the river, Marlow hears Kurtz utter his last words: "The horror… The horror…"

In Apocalypse Now, the river journey is very similar: things start off violent and keep getting worse, before Capt. Willard (the movie's Marlow) finally reaches Kurtz's complex. In the same way that Marlow encounters the dark heart of Belgian exploitation in the form of Kurtz, Willard encounters the dark heart of the Vietnam War in his Kurtz.

For Willard, it's also a journey into his own psyche. He comes to realize that he's more like Kurtz than he'd like to admit. After putting a gravely wounded Vietnamese woman out of her "misery," he says in voiceover:

WILLARD: I felt I knew one or two things about Kurtz that weren't in the dossier.

Come to think of it, Huck Finn is on a journey of self-discovery via a dangerous trip upriver, too. So are Gail and Tom in The River Wild. Ditto Aguirre and the guys in Deliverance.

What is it about rivers?