Riverdead (1970)

A leisurely, beer-filled vacation for a group of partying students turns into a nightmare. After contracting a whitewater river tour, the kids learn that something is stalking them on the legendary Mogollon Rim. Some people believe it to be the mythical Mogollon Monster, but others aren't so sure. No one is safe, including the staff of the river tours, as a force wreaks havoc, slaughtering anything in its path. No phones. No weapons. No chance. The only thing separating the students from certain death is how fast they can paddle.