Considering that Valkyries wasn't in the book, it was orignal to the movie. :-)
Although not much in the movie, if anything, was in the book, the movie was fairly original. :-)
John Milius (the red dawn guy) and Coppola (sp?)wrote the screenplay from Heart of Darkness. There were nods to colonialism (edited out) and the eventual demise of the Brando character/Kurtz along with his conclusions, "exterminate the brutes" v. "nuke'em all" etc.
Also, obviously, the river as the path for the trip...basically a lot of similarities.