SECURITY patrols have been stepped up at Burrator after the dam was used by abseilers.
The climbers stretched a rope across the road on top of the dam near Plymouth to descend the wall.
Anybody seeing the ‘dangerous activity” should report it, says South West Water (SWW).
“Not only were those concerned putting their own lives at risk but also the lives of others,” said a company spokesman.
The news comes as SWW and South West Lakes Trust appealed for visitors to stay safe.
Reports of abseiling and tombstoning – leaping into the water from a great height – were ‘extremely worrying’, the spokesman added.
“We work closely with the trust to make our reservoirs accessible and pleasant for people to visit,” he said.
“However, we would remind visitors that these are operational sites with hidden dangers.
“Every summer, we remind people not to swim in our reservoirs because there are no lifeguarding facilities, they are very deep – often with steep sides – and can be extremely cold.
“Similarly, dams are tall, steep structures and in some cases access to the bottom in an emergency would be challenging.
“Reports of abseiling and even tombstoning at our reservoirs are extremely worrying and we would urge anyone who sees this sort of activity taking place to contact our 24-hour helpline on 0844 346 2020.”
The spokesman added that SWW was increasing security patrols at Burrator.