It flooded the head of the Vyrnwy (Welsh: Afon Efyrnwy) valley and submerged the village of Llanwddyn.

When the reservoir is full, it is 26 metres (84 ft) deep, contains 59.7 gigalitres (13.125×10^9 imp gal), and covers an area of 4.54 square kilometres (1,121 acres),[1] the equivalent of around 600 football pitches.

The valley was chosen because of its size, favourable geology and its source, the River Vyrnwy, having a large water catchment area.

[citation needed] Thomas Hawksley was appointed as engineer-in-chief and prepared the design for a stone dam.

[citation needed] Earlier dams in Britain had been built using great earth embankments to hold back the water.

The dam was the first to be constructed with drainage tunnels designed to prevent a build up of pressure beneath the base.

A power house located under the west tower contains an electrical generator driven by water leaving the reservoir.

Depending on the levels downstream, the reservoir can release from 25 to 45 megalitres (5 to 10 million imperial gallons) of compensation water daily.

[8] in 1946, a fourth pipeline was added south of Oswestry to increase capacity to 227 million litres per day.

In 2013, United Utilities commenced a major refurbishment of the entire 240 km (150 mi) of the LDTM aqueduct, which was scheduled for completion in 2020.

[9] As of 2014 water provision relied on three parallel, 80 km long gravity pipelines, 1.1 metres (42 in) in diameter delivering up to 230,000 cubic metres (50×10^6 imp gal) per day into reservoirs at Prescot, east of Liverpool, which supplied customers in Cheshire and Merseyside.

Sheep, cattle and ponies also graze on the heather, managed by tenant farmers who farm the moorland in accordance with organic agriculture.

There are large wooden picnic benches in the shape of leaves and trees on the west side of the lake at Llechwedd Ddu.

[11] Activities in the area include sailing, hiking on Glyndŵr's Way, rock climbing, cycling, walking and horse riding.

The site was once home to the tallest tree in the UK, a Douglas Fir 63.79 metres (209.3 ft) high.