The narrow point of The Strid near Bolton Abbey where the river narrows from about 30 feet across to about 4 feet across in a very short distance.
The force of the water rushing through here has to be seen to be believed - it's quite a scary place. People who fall in the river here die and the bodies are often not recovered until weeks or months later.
BBC news story about a couple on honeymoon who died here in 1998 can be found here for the more macabre among you:
http://news.bbc.co.uk/1/hi/uk/217851.stm