More than 200 youngsters were found enjoying the warm weather at Ryburn Reservoir in Ripponden yesterday evening.
Some were swimming the water and others had lit fires. There was also litter strewn around the area.
"All were moved on from the area, once some had been made to clean their mess they tried to leave behind," said officers from the Calder Valley team.
"Reservoirs are dangerous. Reservoirs aren't the same as natural lakes, they’re man-made and have large machinery that’s working just below the surface.
"They’re also very cold, have strong currents and might have blue-green algae which causes rashes and severe illness.
"Please discuss with your children, the dangers of entering the water."