Lake Vyrnwy

Lake Vyrnwy/Llyn Efyrnwy

View overlooking Lake Vyrnwy showing the full extent of the lake
Location Wales
Coordinates 52°47′N 3°30′W / 52.78°N 3.50°W / 52.78; -3.50Coordinates: 52°47′N 3°30′W / 52.78°N 3.50°W / 52.78; -3.50
Lake type Reservoir
Primary inflows River Vyrnwy and other small streams
Primary outflows River Vyrnwy
Managing agency Severn Trent Water
Built 1881–88
Max. length 7.64 kilometres (4.75 mi)
Max. width 0.80 kilometres (0.5 mi)
Surface area 4.54 square kilometres (1,121 acres)
Max. depth 26 metres (84 ft)
Water volume 59.7 gigalitres (13.125×10^9 imp gal)
Shore length1 19 kilometres (12 mi)
1 Shore length is not a well-defined measure.

Lake Vyrnwy (Welsh: Llyn Efyrnwy, pronounced [ɛˈvərnʊɨ] or Llyn Llanwddyn) is a reservoir in Powys, Wales. Its stone-built dam, built in the 1880s, was the first of its kind in the world. 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. The Lake Vyrnwy Nature Reserve and Estate that surrounds the lake is jointly managed by the Royal Society for the Protection of Birds (RSPB) and Severn Trent Water, and is a popular destination for days out, and 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

The dam under construction, c.1885
The dam looking east, showing compensation water being released from the reservoir
Water flowing over the crest of the dam
Rhiwargor Falls on the Lake Vyrnwy Estate

Dr George Deacon (1843–1909) began the design of the Vyrnwy Dam in 1879 at the age of 36. In 1890, following Vyrnwy, he founded an engineering practice in London which subsequently became Sir Alexander Binnie Son & Deacon, then Mr Binnie and Partners. Its present-day successor is Black & Veatch.

Dr Deacon was instructed to prepare the parliamentary plans for the scheme in 1879. The dam construction started in 1881 and was completed 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, equivalent to £62,000,000 in 2015.[1] The dam is 45 metres (146 ft) high from the bottom of the valley, and 37 metres (120 ft) thick at the base; it is 355 metres (1,165 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 4.3 metres (14 ft) above the road surface.

Vyrnwy was the first dam to carry water over its crest instead of in a channel at the side. At the bottom of the dam is a body of water known as a Stilling Basin, this is necessary to absorb the energy when the water flows over the crest and into the valley, and to stop the water eroding the foundations of the dam.

A power house located under the west tower contains an electrical generator driven by water leaving the reservoir. Before mains electricity arrived in the 1960s this was the area's only source of power.

The west and east Towers release compensation water by huge valves, which are controlled by Severn Trent Water. This water flows into for the River Vyrnwy, which would otherwise dry out unless in flood. Depending on the water levels downstream the reservoir can release anything from 25 to 45 megalitres (5,500,000 to 9,900,000 imp gal) of compensation water per day. This flow is measured by the Environment Agency at a weir few hundred metres downstream.

Earlier dams in Britain had been built using great earth embankments to hold back the water. This new type of stone dam would change the face of the Welsh landscape over the coming years. The next stone dams to be built in Wales on an even bigger scale than Vyrnwy were those built in the Elan Valley.[2]

Straining tower and aqueduct

A sideview of the dam (2009)

Approximately 1,200 metres (0.75 mi) from the dam is the reservoir's straining tower. Standing only 30 metres (98 ft) from the shore, its purpose is to filter or strain out material in the water with a fine metal mesh, before the water flows along the aqueduct to Liverpool. Its architecture represents Gothic revival, built at the same time as the dam. The tower as a whole is 47 metres (154 ft) tall, 32 metres (104 ft) of which is above water, and is topped with a pointed copper-clad roof, coloured light green.

The Gothic revival straining tower

The water from Lake Vyrnwy is carried 109 kilometres (68 mi) to Liverpool in the Vyrnwy Large Diameter Trunk Main (LDTM) aqueduct. This originally consisted of two pipelines, made largely of cast iron. To help maintenance work on the 2.7 metres (9 ft) diameter cast-iron tunnel which took the aqueduct under the Mersey, riveted steel piping was also used. This was an early use of the material which was to become the norm for trunk water-main piping.

The original aqueduct was constructed across the valley from the reservoir between 1881-92. It crosses the valley floor near Penybontfawr and then runs north of Llanrhaeadr-ym-Mochnant and Efail-rhyd on the north-east of the Tanat Valley. The aqueduct is largely hidden from view although there are some visible surface features including air valves, the Cileos valve house, the Parc-uchaf balancing reservoirs, and a deep cutting to the west of Llanrhaeadr-ym-Mochnant.

Brick and concrete-lined tunnels carried pipes at Hirnant, Cynynion and Llanforda, and a fourth later added at Aber so that the Hirnant tunnel could be made accessible for maintenance. The first section of a third pipeline was laid in 1926-38 using bituminous-coated steel. To increase capacity, a fourth pipeline was added in 1946.

Re-organisation of the pipe crossings beneath the Mersey and the Manchester Ship Canal was undertaken in 1978-81. The current provision relies on three pipes 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. In 2013 United Utilities commenced a major refurbishment of the LDTM aqueduct, which was scheduled for completion in 2020.[3]

The lake/reservoir

The reservoir is Severn Trent Water's largest. When 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),[4] the equivalent of around 600 football pitches. The lake has a circumference of 19 kilometres (12 mi) with a road that goes all the way around it. Its length is 7.64 kilometres (4.75 mi). On a clear day the lake, along with many others in North Wales, can be seen from space.

311 brooks, waterfalls and rivers flow into the lake and are named after the mountains or hillsides they flow from. Some are no more than a trickle, while others cascade down the mountains. The main ones, clockwise from the west side of the dam, are named Afon Hirddu, Eunant, Afon Eiddew, Afon Naedroedd, Afon Cedig and Afon Y Dolau Gwynionew.

On the northern edge of the lake is a small hamlet called Rhiwargor where the rivers Afon Eiddew and Afon Naedroedd meet. Up the valley of Afon Eiddew is an impressive waterfall, one of the largest surrounding the lake. Known locally as Pistyll Rhyd-y-meincau, it is commonly known as Rhiwargor waterfall.

In 1889, shortly after completion, the lake was stocked with 400,000 Loch Leven trout.

The lake continues to supply Liverpool with fresh water. It is the water source used in the manufacture of Bombay Sapphire gin.

River Vyrnwy

Main article: River Vyrnwy

The River Vyrnwy (Welsh: Afon Efyrnwy) runs from the Welsh mountains; its sources are from many and varied brooks and tributary rivers from around the lake. However, since it was flooded, the river starts at the foot of the dam and flows east towards England, eventually finding its way to Shropshire where it converges with the River Severn near the village of Melverley on the Welsh border. The river runs for 63.9 kilometres (39.7 mi); the last 13 kilometres (8 mi) forms a natural boundary between England and Wales. The River Severn then takes its course though England to the Bristol Channel.

Nature reserve and conservation

Lake Vyrnwy is a designated Nature Reserve. The RSPB has several bird hides around the lake, where a number of rare species of birds are known to be breeding, including the peregrine falcon, the pied flycatcher, the redstart, the siskin and the wood warbler. Every spring they host a dawn chorus tour.

Around 90 species of bird have been recorded as breeding on the reserve, and six species of bat, including the pipistrelle and brown long-eared bat. Butterfly species include purple hairstreaks, commas and peacocks. Dragonflies include golden ringed, common hawker and four spotted chaser.

Heather moorland that grows on the mountains around the lake is now being restored. This restoration of heather moorland is becoming increasingly common in Britain. In most moorlands, heather is usually burnt, cut and the seeds collected to be sowed where the heather has gone. Burning at the Lake Vyrnwy moorland is no longer carried out, as the burning of heather can have negative consequences for water management (namely water colouration). Management of the moorland helps improve the habitat for red grouse and the short-eared owl.

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

Broadleaf trees are being planted to replace coniferous trees, and man-made features such as hedgerows and dry-stone walls are also being restored, and wildflower areas are being restored to help insects, birds and other wildlife.


Llanwddyn, the community surrounding Lake Vyrnwy, had a population of 257 in 2011, according to the census;[5] a 17.1% decrease since the 310 people noted in 2001.[6]

The 2011 census showed 38.1% of the population could speak Welsh, a fall from 60.3% in 2001.[7]


Wood sculpture

Llanwddyn has had since 1995 a sculpture park in the valley below the dam, containing many wooden carved works. There are large (beyond imagination) wooden picnic benches in the shape of leaves and trees on the west side of the lake at Llechwedd Ddu. Near the old village on the beach is a sculpture of dolphins which, when the lake rises in a flood, appear to be jumping out of the water. Several totems are carved into standing trees and re-erected fallen trunks.[8]


Activities in the area include sailing, hiking on Glyndŵr's Way, rock climbing, cycling, walking and horse riding.[9] The Lake Vyrnwy Half Marathon is conducted annually.

The RSPB have seven waymarked trails ranging from 2 to 9 kilometres (1 to 5.5 mi) in a range of habitats.

Tallest tree

The site was, until recently, home to the tallest tree in the UK, a Douglas Fir 63.79 metres (209 ft) high. This tree was damaged in stormy weather and had to be felled. A nearby Douglas Fir has now inherited the title of 'Tallest tree in Wales', at 60.62 metres (199 ft) tall.[10]


  1. UK CPI inflation numbers based on data available from Gregory Clark (2016), "The Annual RPI and Average Earnings for Britain, 1209 to Present (New Series)" MeasuringWorth.
  2. "The Elan Valley dams". Powys Digital History Project. Powys Heritage online. 2002. Retrieved 3 July 2013.
  3. "Vyrnwy water pipe works". United Utilities. Retrieved 3 July 2013.
  4. "Lake Vyrnwy". The Practical Engineer. Technical Publishing Company. V. 1891. Retrieved 11 June 2014.
  5. "Area: Llanwddyn (Parish)". Office for National Statistics. 30 January 2013. Retrieved 11 August 2015.
  6. "Area: Llanwddyn (Parish)". Office for National Statistics. 18 November 2004. Retrieved 11 August 2015.
  7. "2011 Census results by Community". Welsh Language Commissioner. 2015. Retrieved 11 August 2015.
  8. "The Sculpture Park at Vyrnwy". Glyn-yr-Aur. Retrieved 3 July 2013.
  10. "Neighbour inherits 'tallest tree' title". Retrieved 15 March 2014.
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