A photographer captured the latest incident as one canoeist slid 300ft down the spillway at Llyn Brianne reservoir in Carmarthenshire, near the borders of Ceredigion and Powys.
Welsh Water said the practice was dangerous and such activities were banned at the reservoir.
An onlooker said: “I always stop and look anyway and I saw four people ready to do the spillway.
“There were a lot of people watching at the top car park.
“It looked pretty scary.”
Llyn Brianne was completed in 1972 and supplies drinking water to a large part of south Wales.
The reservoir is up to 272 ft (83m) deep and holds more than 13 billion gallons (60 billion litres) of water.
A statement from Welsh Water, the owner of the site, said it “strictly forbids” water-based activities at the reservoir and would take “appropriate action” against anyone in breach of this restriction. “Reservoirs can be dangerous for various reasons and those involved in water sports in inappropriate locations, such as at Llyn Brianne, put themselves and others at unnecessary risk,” said a spokesman.
Richard Harvey from the Welsh Canoeing Association said he could not comment on the group involved, the locality or the legality of their activity.
“We encourage people to paddle responsibly and in the realm of the law,” he said.
“If they go too far we do not condone it.”