Deliverance

Not marketed as horror and the founding text of an entire horror subgenre.

Four Atlanta businessmen canoe a river before a dam floods the valley. They meet people who live there.

Boorman shot it in sequence on the Chattooga, with the actors doing their own paddling and no stunt doubles for most of it. Burt Reynolds cracked his tailbone going over a fall. Ned Beatty was in real danger in the water more than once.

The assault sequence entered the culture so completely that the banjo tune, which is a scene about a moment of genuine connection, is now shorthand for menace.

What the film is actually about is the dam. These men are tourists in a place that is about to be erased and the people being erased with it know exactly what they are.

Everything in backwoods horror is downstream. None of it is as well made.