 Work on the William Girling Reservoir was started in 1938, with John Mowlem & Co being the contractor, but owing to technical problems, and the intervention of World War II, the reservoir was not finished until 1951.The reservoirs are a 391.3 hectare biological Site of Special Scientific Interest, of which 316.3 hectares are in London and 75 hectares in Essex.They are comparatively shallow and provide open water habitat for wildfowl, gulls and waders.