Dredging and gravel extraction continued until the 1940s. In 1947 Rickmansworth and Uxbridge Valley Water Company purchased Stocker's Lake (an area of 93.7 acres) for £5000 as the dredging agreement had expired. This was to protect the aquifer and pumping sources at Stocker's and Springwell. The company became Rickmansworth Water Company in 1985 and later part of Three Valleys Water Company, eventually Affinity Water, the present owners.
In the 1970s Stocker’s Lake became a nature reserve and day to day management was assigned by the water company to Herts and Middlesex Wildlife Trust. The main reason was that the lake had started to support nationally significant numbers of wildfowl, especially Shoveler.
Friends of Stocker’s Lake was started in May 1990, by voluntary warden Richard Drew and his friend Jol Mitchell, FoSl chairman 1990-2003. Their idea was to use 'friends' annual subscriptions to fund the construction of bird hides and other projects around the lake. And so it has proved. By 2016 FoSL had spent more than £70,000 on the reserve, with only a little of this coming from grants.