 Construction of the King George V Reservoir was started in 1908 and completed in 1912.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.