In the 70's Prue and Piper attended Camp Skylark during the summer then the drownings started which lead to the death of their mother.
The Charmed Ones
Years later in the 90's an adult Prue uses the location as a place to think. She witnesses a drowning then she and her sisters decide to stop it. Sam tries to stop them, but they are persistent. He eventually uses memory dust to get them to stay back, but they regain their memories and go back to vanquish the water demon. Before Mrs. Johnson's death she had planned to reopen the camp but the drownings started again. She decided not to but she died before she could shut the camp down so several children showed up, so Phoebe used memory dust on them to keep them out of the water until the demon was vanquished.