Savage Water (1979)

A group of holidaymakers have booked themselves a dream trip with the Wild West White Water River Boat Company. Their journey will take them along the great Colorado so they can experience first hand the beauty of the Grand Canyon and the ferocity of the water crashing over the rapids. Once they are well away from civilisation, things take a turn for the worse as it's realised that someone on board has their own reason for wanting to be stranded in the Canyon without interference from the authorities. Before long, the group begin getting bumped off one by one by an unseen maniac with a hunting knife and a murderous agenda. It seems that the killer wants to turn the mighty Colorado into a river of blood… (IMDb)