Lake Vyrnwy Nature Reserve and Estate (Welsh: Llyn Efyrnwy) ([pronounced [E]VURN-OI]) is an area of land in Powys, Wales, surrounding the Victorian reservoir of Lake Vyrnwy. Its stone-built dam, built in the 1880s, is 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 reservoir is Severn Trent Water's largest. When full, it can take as much as up to 59,666 megalitres (1.3125×1010 imp gal), and it covers an area of 4.53 cubic kilometres (1,120,000 acres) of land, the equivalent of 600 football pitches. The lake has a circumference of 11 miles (18 km) with a road that goes all the way around it. Its length is 4.75 miles (7.64 km). On a clear day the lake, along with many others in North Wales, can be seen from space.

There are 31 streams, waterfalls, and rivers that flow into the lake. Some are no more than a trickle, while other waterfalls cascade down the mountains. The 6 rivers that flow into the lake are all named respectively to the mountains or hillside it flows from.

Source: www.wikipedia.org