Kayakers are filmed plunging down the spillway at Llyn Brianne in mid Wales. All photos by James Davies, Swansea
The spillway runs 300ft (91m) down the side of the dam.
"It looked pretty scary." An onlooker said there were a number of people watching from the car park.
Welsh Water, which owns the site, says the stunts put the kayakers and other people at "unnecessary risk".
One of the kayakers has a rough landing at the valley floor.