Lake Vyrnwy Nature Reserve and Estate (Welsh: Llyn Efyrnwy ) 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 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. The Reserve is designated as a National Nature Reserve, a Site of Special Scientific Interest, a Special Protection Area, and a Special Area of Conservation.
The dam construction started in 1881 and was completed seven years later in 1888. It was the first large stone-built dam in the United Kingdom, and is built partly out of great blocks of Welsh slate. When built it cost £620,000, which today is around £22,000,000. The dam is 44 metres (144 ft) high from the bottom of the valley, and 39 metres (128 ft) thick at the base; it is 357 metres (1,171 ft) long and has a road bridge running along the top. It is decorated with over 25 arches and two small towers (each with four corner turrets) rising 4 metres (13 ft) above the road surface.
