Police are warning teenagers against a new craze - of playing "chicken" with canal boats.
Up to 30 youngsters are gathering at Swarkstone lock, Derbyshire, on the Trent and Mersey canal, and plunging from bridges near the slow-moving craft.
Officers say the suction created when the lock gates open could drag the swimmers underwater.
Police have condemned the game after being called to deal with gangs who intimidate boat owners.
Roy Bowyer, who visits the lock regularly, said: "They bring all the gear down - cans, bottles and rubbish because when they've left it's in a dreadful state.
"They're an absolute menace jumping into the lock. They sit on the bridge wall over the canal, wait for the boats showing their noses and jump.
"They're so menacing and the canal is only 4ft 6ins deep."
Pc Clare Grice said: "These teenagers have been wanting to jump in front of the barges which is not only a danger to them but to themselves.
"With it being so close to the lock it's the force of the lock as it's opened which can drag someone into the lock.
"If a big group of youths is seen then people must ring 999 and we'll have somebody down here as soon as possible."