Lake Vyrnwy Nature Reserve and Estate (Welsh: Llyn Efyrnwy, pronounced [ɛˈvərnʊɨ]) is an area of land in Montgomeryshire, Powys, Wales, surrounding the Victorian reservoir of Lake Vyrnwy. Its stone-built dam, built in the 1880s, was the first of its kind in the world. The nature reserve and the area around it are jointly managed by the Royal Society for the Protection of Birds (RSPB), and Severn Trent Water. It was built for the purpose of supplying Liverpool and the districts later designated as Merseyside with fresh water. It flooded the head of the Vyrnwy valley and submerged the small village of Llanwddyn. Today it is a popular retreat, for people in the West Midlands and Merseyside for days out, and also for ornithologists, cyclists, and hikers.