With temperatures touching 30C, the reservoir might seem tempting, but taking a swim could lead to tragedy.
People have been swimming in Farnley and although nobody has yet been harmed this summer, the story could easily have been different.
Emily Brady from Yorkshire Water said: “Most people think reservoirs are safe places to swim, but they pose a huge risk which could lead to loss of life. They are often colder than rivers and this can result in cold water shock that can lead to hyperventilation, increased blood pressure, breathing difficulties and heart attacks plus water temperatures remain just as cold in summer as in winter.”
West Yorkshire Fire Service, Kirklees District Commander Toby May, said: “In the last two years, firefighters have attended over 70 water rescue incidents in various types of waterways across West Yorkshire.
“We realise the appeal of taking a dip during the hot summer months, but the cool water of a reservoir can be dangerously deceiving and you can very easily get into difficulty as cold water shock takes hold.
“Please take care around open water - particularly as school summer holidays start, it is vital that young people take our safety messages on board when they are out and about.”