He was white, in his late 20s and was wearing a green rain jacket and baseball cap.
South Yorkshire Police said: “The man was spotted by a passenger in a vehicle as it travelled along Rivelin Valley Road in the direction of Manchester Road. As the vehicle passed the water park, the victim saw a man stood on the bridle path with his trousers pulled down committing the lewd act.
“Officers attended the area, but were unable to locate the man.”
Anyone with information should call 101 and quote incident number 329 of June 7.
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