Elan Valley Lakes

Craig Goch dam and reservoir in autumn © Hawlfraint y Goron / © Crown copyright (2022) Cymru Wales

THE Elan Valley reservoirs are a chain of man-made lakes created from damming the Elan and Claerwen rivers within the Elan Valley in Mid Wales. The reservoirs, which were built by the Birmingham Corporation Water Department, provide clean drinking water for Birmingham in the West Midlands of England.

The dams are the biggest draw for visitors to the Elan Valley and they offer a wonderful year-round backdrop for cyclists, walkers, and photographers. With the exception of Dol y Mynach, all the dams are also accessible by car.

There are four dams on the river Elan; Craig Goch, Pen y Garreg, Garreg Ddu, and Caban Coch. The river Claerwen has the newest, and largest Claerwen dam followed by the unfinished Dol y Mynach dam.

Work to build the Elan Valley reservoirs was originally undertaken because the rapid growth of the industrial city of Birmingham in the late 19th Century had led to a lack of available clean water. Numerous outbreaks of disease prompted Birmingham City Council to petition the British Government which passed the Birmingham Corporation Water Act in 1892. It allowed the corporation to acquire by compulsory purchase all the land within the water catchment area of the Elan Valleys.

Thousands of navvies and their families lived in the purpose-built Elan Village during the construction of the first four dams at the turn of the 20th Century. In 1952, the Claerwen dam was opened by Elizabeth II in one of her first official engagements as monarch.

The reservoirs are now owned and managed by Dŵr Cymru Welsh Water. The water filtration works further down the valley is run by Severn Trent Water.

During World War II, in 1942, the Nant-y-Gro dam, which had been built to provide water for the navvies’ village, was used by Barnes Wallis to prove his theory that an underwater explosion could create sufficient hydrostatic pressure to collapse a dam wall. The test proved successful as a significant breach in the middle of the 11-metre (36ft) dam wall was caused by a charge placed at its base.

The lessons learned at Nant-y-Gro green-lit the development of the ‘upkeep’ bouncing bombs. They were used in the 1943 Dam Busters raid when RAF Bomber Command breached and damaged several dams in the Ruhr Valley, Germany.

The remains of the breached Nant-y-Gro dam are in much the same state as it was left in 1942, although partly obscured by trees.

After the Dam Busters raid, steel cables were strung across the valley above the Caban-coch reservoir as a precautionary measure against German reprisals.

The Elan Valley in rural Mid Wales has been inspiring visitors for centuries. Inhabited since the stone age, the area became known for mining, religious sites and for taking part in the Rebecca Riots of the 1840s. Today the valley is famous for the spectacular dams and scenery. The nearest large town is Rhayader, on The Cambrian Way, snaking through Mid Wales.

A highlight edit from a 45-minute promotional film for the Elan Valley in Mid Wales, presented by wildlife expert Iolo Williams Credit: Andrew Goode TV

Over 80 per cent of the valley is designated as Sites of Special Scientific Interest (SSSI), home to reservoirs, aqueducts, hundreds of animals and plenty of opportunities to have a memorable and adventurous time.

Elan is a beautiful and unspoilt area made even more compelling by the dams and reservoirs which together create a wonderful, living landscape. The views are stunning and you are never far away from points of interest.

Elan is a haven for wildlife, one of the most important sites in Wales, and there is always something to warm the heart throughout the year. Visitors marvel at the engineering enterprise of the Victorians in building the dams and the railways which served their construction. A team of Rangers are on hand to provide more background and history.

Pen y Garreg © Hawlfraint y Goron / © Crown copyright (2022) Cymru Wales

The dams, reservoirs and 73-mile aqueduct of the Elan Valley were built a hundred years ago in an epic feat of civil engineering set within an area of outstanding scenic beauty.

The 70 square miles of moorland, bog, woodland, river and reservoir are of national importance for their diversity of lower plants (mosses, liverworts and lichens) and the Estate is the most important area for land birds in Wales.

Most of the 180 square kilometres of the Elan Estate is covered by 12 separate Sites of Special Scientific Interest. Within the Estate is the Claerwen National Nature Reserve, 800 hectares of upland plateaux with gently rolling hills covered with acid grassland and, in parts, blanket bog on a mantle of peat. Grazing has been reduced on the reserve to protect species like bog mosses, bog rosemary, cotton grasses and heather. This bleak upland provides breeding or feeding ground for scarce birds like the dunlin and golden plover.

The Estate has been awarded an International Dark Sky Park Award and it is indeed filled with a wealth of nocturnal wildlife which thrives under these very dark skies.

For well over 100 years sheep farming and the supply of water for public consumption have happily co-existed on the Elan Estate and these activities have maintained the special landscape and nature conservation value of the area at a time when large areas of the uplands have either been afforested or industrialised through wind farms.

As you breathe in the tranquillity of Elan it can be hard to imagine that this has been a place with such long and varied history. 4,000 years ago Stone Age people made Elan their home within the forests of oak, birch and hazel. Later arrivals included Celts and Romans. More recently, Elan’s resources have attracted mining interests. Coming up to date, of course, Elan’s dams and reservoirs continue to provide water to significant populations.


1892: Act of Parliament passed allowing the purchase of the watershed land, for the building of the dams.

1893: Work begins on the building of the Elan Valley dams.

1904: Elan Valley dams completed and opened by King Edward VII and Queen Alexandra.

1914-1918/1939-1945: World Wars I and II. Dams are specially guarded against attack.

1946: Work begins on Claerwen Dam.

1952: Claerwen Dam is opened by Queen Elizabeth II as one of her first duties as monarch.

1965: The first Site of Special Scientific Interest is designated on the Estate.

1974: The individual water companies are designated and Dŵr Cymru Welsh Water is given responsibility for the Elan Estate, dams and reservoirs.

1985: Elan Valley Visitor Centre opens.

1989: Elan is included in the Cambrian Mountains Environmentally Sensitive Area.
The Elan Valley Trust is established after privatisation of the water companies to protect the wildlife of the Estate and encourage public access and understanding.

1995: Elenydd-Mallaen is designated a Special Protection Area under the European Wild Birds Directive.

1997: New Visitor Centre extension opens.

2002: Claerwen Dam celebrates its half-centenary with the dam being opened for visitors to look inside.

2004: Elan Valley Dams celebrate their centenary, with a series of special events.


River Elan Dams

Craig Goch is the highest upstream of the series of dams in the Elan Valley. It is located at a height of 1040 feet (317 metres) above sea level. As with all the dams, work started with the arrival of the railway line at the site. In the case of this dam the line had the furthest to go and a rocky outcrop had to be blasted and dug through on the route to the site. Work on excavating the foundations for a secure base for the structure started in July 1897, some three years after the start of work on the lowest dam at Caban Coch.

Craig Goch is seen by many as the most attractive of the dams, with an elegantly curved retaining wall and a series of arches carrying a narrow roadway across the top of the dam. It has a domed valve tower and the structure is typical of the ‘Birmingham Baroque’ style of much of the waterworks scheme.

Garreg Ddu in the lower Elan Valley serves a dual role. It is a low, completely submerged dam which plays a vital role in maintaining a constant supply of water to Birmingham. It also supports masonry pillars carrying the access roadway to the neighbouring valley of the River Claerwen. Garreg Ddu holds water back on the upstream side so that water can always be extracted at the Foel Tower. The bottom of Caban Coch Reservoir is too low to allow water to be gravity-fed to Birmingham. Extraction from here would require pumps. The submerged dam played its intended role of maintaining the flow to Birmingham  in September 1937 after a similar period of exceptional drought when the water levels in the Elan Valley dropped alarmingly.
The original road leading to this valley was to be lost, along with many original buildings, with the completion of the Caban Coch dam and the subsequent flooding of the two valleys.

Pen y Garreg is the third dam up the Elan Valley. The viaduct at Garreg Ddu further downstream does not resemble the other dams since the dam part of the structure is not visible above the water surface in normal conditions. Pen y Garreg is unusual in that it houses an access tunnel to the central tower which is lit by apertures in the downstream side of the dam.

Caban CochThe lowest of the dams in the sequence of four built in the valley of the Elan River is Caban Coch (pictured above). It is the simplest and most functional in appearance of all the dams, resembling a natural waterfall when the reservoir is full and the dam is in full spate with water pouring over the dam wall.

Eustace Tickell, one of the senior engineers on the waterworks scheme, wrote of this dam before it was completed: “… in time of flood, when the storm water rushes over the crest and falls to a depth of over 120 feet, the dam at Caban Coch will present the appearance of a magnificent waterfall”.

Caban Coch contributes to the supply of water to Birmingham when water levels are normal, but it also provides compensation water to ensure that adequate flow is maintained in the Elan and the Wye downstream from the dams. There are identical stone buildings on either side of the river just below the dam wall which house electricity generating turbines and valves and sluices to adjust the amount of compensation water released downstream.

River Claerwen Dams

The severe drought of 1937 served to give warning of the increasing need for much greater water storage capacity. The three dams proposed for the Claerwen Valley as part of the original Elan Valley waterworks scheme of 1892 had not been built, apart from the base of the dam at Dol y Mynach which had to be constructed early because of its location below the top water level of the Caban Coch Reservoir. Proposals for a large new dam in the upper Claerwen Valley were at an advanced stage by early 1939, but the Second World War meant that the demands of wartime production itself put even greater strains on the existing water supplies. The increasingly urgent calls for a new dam and reservoir on the Claerwen were to be reactivated soon after the end of the war. Progress in civil engineering techniques and in mechanisation, however, meant that much larger dams could be built by this date.

It is a measure of its size that Claerwen dam was to create a reservoir which holds almost as much water as the combined total of the three earlier dams built in the neighbouring Elan Valley. The new dam is 184ft (56m) high and 1167ft (355m) long. Claerwen dam was designed to be in keeping with the appearance of the much older structures nearby. Although built in concrete, the huge dam was faced with dressed stone at considerable extra cost in materials and labour. The construction of the Claerwen Dam, the last of the dams in the district, took six years, using a workforce of 470. The improved techniques and mechanisation of large-scale civil engineering projects meant that large numbers of manual workers were no longer needed.

The Claerwen Dam was ceremonially declared open by the newly-crowned Queen Elizabeth in October 1952, in one of the first official engagements of her reign.

Dol y Mynach is the unfinished dam. The original 1890s scheme for the sequence of dams and reservoirs in the Elan and Claerwen valleys included provision for three dams on the River Claerwen, which were intended to be constructed later when additional water supplies were needed for Birmingham. Caban Coch dam was to create a reservoir with a top water level which would be above the level of the foundations of the dam at Dol y Mynach, the lowest of the planned three on the River Claerwen. It was therefore necessary to build the base of the Dol y Mynach dam at the same time as the other dams in the adjacent valley of the River Elan.  Huge blocks of stone, weighing anything up to 10 tons set in concrete which form the solid core of the huge structure, can be seen between the outer faces of dressed masonry.

Elan Valley Trust, Elan Estate Office, Elan Village, Rhayader, Powys LD6 5HP. Telephone: 01597 810449. E-mail: [email protected]. Website: elanvalley.org.uk

(sources include Elan Valley Trust, Visit Wales, Wikipedia)