Walthamstow Wetlands is a 211-hectare (520-acre; 2.11 km2) nature reserve in Walthamstow, east London, adjacent to the historic Essex-Middlesex border on the River Lea. It is focused on the Walthamstow Reservoirs, built by the East London Waterworks Company between 1853 and 1904 as part of the Lee Valley Reservoir Chain. The site is one of the largest urban wetland nature reserves in Europe and is particularly important for wildlife due to its position within the Lee Valley. It serves as a byway for migrating, wintering and breeding birds. Visitors can freely access the site's natural, industrial and social heritage in one of the capital's most densely populated urban areas.