Answer: Deliverance
In "Deliverance", Burt Reynolds, Jon Voight, Ned Beatty and Ronny Cox decided to spend a weekend canoeing down the Cahulawassee River before the river disappears under a flood that will be released when a dam will be destroyed. They mixed it up with some very, very disturbing locals over the course of the weekend, and their lives were changed for the worse (those who survived, anyway). James Dickey wrote the novel and had a small part in the movie.
From the film's trailer: "These are the men. Nothing very unusual about them. Suburban guys like you or your neighbor. Nothing very unusual about them until they decided to spend one weekend canoeing down the Cahulawassee River. Ed Gentry - he runs an art service, his wife Martha has a boy Dean. Lewis Medlock has real estate interests, talks about resettling in New Zealand or Uruguay. Drew Ballinger - he's sales supervisor for a soft drink company. Bobby Trippe - bachelor, insurance and mutual funds. These are the men who decided not to play golf that weekend. Instead, they sought the river."