The ponds were lined with concrete in 1968 to stop them drying out. They are periodically emptied to clean and repair. The last time they were fully emptied was in 1996. As a record, here’s a brief video uploaded to the Secret Carshalton YouTube channel.
I was fishing in that channel in the video with my little brother I have been hunting for the photo of us kneeling down fishing for years the picture was taken by the Carshalton and Wallington Advertiser.