 In the mid 19th century the East London Waterworks Company purchased the Coppermill at Walthamstow and modified it to drive a water pump to assist in the building of reservoirs on nearby marshland in the Lea Valley .The company built a series of reservoirs which were High Maynard Reservoir, Low Maynard Reservoir, five linked numbered reservoirs making the Walthamstow Reservoirs, the East Warwick Reservoir and the West Warwick Reservoir.In 1872 the Lambeth Waterworks Company moved upstream on the Thames to Molesey, followed by the Chelsea Waterworks Company.