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The Elan Valley Reservoirs

The ‘Welsh Lake District’ – an impressive network of reservoirs and dams in the hills to the west of Rhayader. They are situated in an area of outstanding natural beauty and are owned by Dŵr Cymru - Welsh Water. After passing through the system of reservoirs the river Elan joins the Wye.


View The Elan Valley Reservoirs in a larger map
The Source of the River Wye
The River Wye at Pant Mawr
The River Wye at Llangurig
The Wye Valley between Llangurig and Rhyader
Bryn Titli wind farm
Rhyader - the first town on the River Wye
The Elan and Caerwen reservoirs
Gigrin Farm Red Kite Feeding Station

Video: The Elan Valley Dams (5 minutes 30 seconds)

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This video shows you all of the reservoirs and dams in the Elan and Claerwen Valleys.

This tour takes about five minutes.

It is a section from the DVD 'A Year on the Wye' - Click here for more details.

The River Elan

The River Elan rises on the hill of Geifas, south of the lead mining valley of Cwmystwyth. It flows through the wilderness of Mid Wales. Probably the only wilderness in the UK south of Hadrian's Wall. After passing under the tiny road bridge of Pont Ar Elan it flows into the Craig Goch Reservoir - first of four reservoirs before it continues as a river, finally flowing into the River Wye.


Why were the reservoirs built here?

In the 19th Century, as a result of the Industrial Revolution, Birmingham was growing rapidly. The supply of clean water became inadequate for the city. Generally, people depended on wells that often became contaminated with sewage. This directly led to epidemics of diseases attributable to unclean water such as cholera, typhoid and diarrhoea.

Birmingham City Council set about finding a supply of fresh clean water for the city. A site had already been identified by one James Mansergh. He recommended the Elan and Claerwen Valleys because:

  • The area received a more than average annual rainfall of 1830mm.
  • Narrow river valleys made building any dams easier.
  • The area was underlain with impermeable bedrock – this would prevent water from seeping away.
  • The area was of a higher altitude that Birmingham, so water could flow to the city without any need for pumping.
  • Finally, being sparsely populated moorland, the area was relatively cheap and easy to clear.


Building the dams

In 1893 work began on building the reservoirs in the Elan Valley – the first phase. This required 100 occupants to move, however only landowners received any compensation.

The construction project required its own infrastructure. Three years were taken building a railway to transport workers and the massive volume of building material. A village of wooden huts sprung up on the site of the present Elan village. Ironically a small reservoir, contained by the Nant-y-Gro dam was built to provide water for the workmen. In all about 50 000 worked on the first phase of the project, which cost £6 Million.

Edward VII and Queen Alexandra opened the Elan Valley reservoirs on the 21st July 1904, and water started flowing towards Birmingham.

The First and Second World Wars delayed the second phase – the Claerwen Valley. One advantage of the delay were the advances in engineering that had meanwhile taken place. Rather than three dams, just one larger dam was now needed.

Unlike the older dams, that were rock filled, the Claerwen dam was concrete. However it was faced in stone to be in harmony to the other dams. Just after the Second World War British stonemasons were busy putting to right damage to buildings especially in cities like London. So Italian Stonemasons worked on the project.

Queen Elizabeth II opened the Claerwen Dam on 23 October 1952.



The maximum capacity for each reservoir is given in megalitres:

  • Craig Goch – 9,222
  • Penygarreg – 6,055
  • Caban Coch with Garreg Ddu – 35,530
  • Claerwen – 48,300 .


The Craig Coch Reservoir

Craig Coch is the first reservoir the Elan flows in to, because it is the first the river meets - and highest above sea level - it is referred to as the Top Dam.

However work began on this dam later than any others in the first phase: three years after construction of the lowest dam had started. This is because - for raw materials and the workforce - work depended on the railway built to support the construction. Only after three years did that reach the site of the dam. Work commenced in June 1897.

In the 1970s the Craig Coch dam was threatened by a massive new reservoir, the dam of which would have dwarfed Craig Coch and in all probability obliterated it. Fortunately for the character of the reservoir and the area, this proposal was finally abandoned.


The Penygarreg Reservoir

Often referred to as the 'Middle Dam' - when in fact it is the second of four dams (the lower Carreg Ddu dam not being obvious - see below). Work commenced on the Penygarreg dam in April 1895.

Masonry pegs were built in to the wall of the dam that during construction supported a cantilevered trackway to carry materials across the face of the dams to cranes. Also an access tunnel to the central tower - this is lit by apertures in the wall of the dam. These apertures have sills above them, so when water is cascading down the dam, it is thrown clear of the apertures.


Electric Power

In the 1990s the potential of utilising the dams to produce electricity was realised. Turbines were installed at the base of each dam and in the Foel Tower.

The electrical output for each dam is:
Claerwen - 1680 kilowatts
Craig Goch     480 kilowatts
Pen y Garreg     810 kilowatts
Caban Coch     950 kilowatts
Foel Tower     300 kilowatts

This gives a total maximum output of 4.2 megawatts – more than enough to supply 4000 homes.


The Carreg Ddu Reservoir

The Carreg Ddu reservoir appears to be a single extension of the Caban Coch reservoir, the only separation being a roadbridge. This is not quite true - the Carreg Ddu reservoir is - unless conditions are exceptionally dry - permanently submerged. The purpose of the submerged dam is to maintain a minimum level for the Carreg Ddu reservoir, so that water can continue to be drawn off for Birmingham.

Above the submerged Carreg Ddu dam are masonry pillars that emerge from the water to carry the access road to the Claerwen Reservoir across the water.

On the Western side of the dam is the 'new' Baptist Chapel built by Birmingham Corporation. This was built to replace the church of Nantgwyllt which lay further down the valley and was submerged by the Caban Coch reservoir.


The Foel Tower and the Elan Aqueduct

Situated in the Carreg Ddu reservoir is the elegant Foel Tower. This contains valves and cylinders used to draw water off for Birmingham.

The Foel Tower is 52 metres higher than the Frankley reservoir in Birmingham – 118 km (73 miles) away. This means the treated water need not be pumped on its journey to Birmingham, but flows by gravity. This is through the Elan aqueduct.

The Elan Aqueduct falls from the Foel Tower to the Frankley Reservoir at a gradient of 1:2300. The gentle fall means that water from the Carreg Ddu reservoir travels at less than 1.5 km per hour, and takes one and a half days to reach Birmingham.



The Caban Coch Reservoir

Approaching from the town of Rhayader, the dam of this reservoir is the first sight of the Elan valley complex visitors will see. When water is cascading down the face of the dam, this makes a spectacular introduction.

For the same reason that the Craig Goch reservoir was the last to commence construction, the Caban Coch reservoir was the first - because the railway that supplied vital construction material reached the site of the Caban Coch dam before any other. This was in 1894 when construction began.

While water levels are normal, the Caban Coch reservoir contributes to the water supply. However when levels are low it maintains the flow of the river Elan, contributing to the flow of the River Wye, after the Elan joins it at Rhayader.


The Dambusters Connection

The ‘Dambuster’ Raids on the Ruhr Valley Dams, using ‘bouncing bombs’ are well known. Barnes Wallace had designed the bouncing bomb, but needed to test whether the explosives to be placed in the bombs would breach a dam. The Nant-y-Gro dam – originally built to provide water for the Elan Valley construction force – provided an ideal testing ground. These tests culminated in the  Nant-y-Gro dam being breached in 1942. The test confirmed the necessity of an explosive charge being delivered below water level and against the inside wall of the dam.

After the Dam Buster raids, steel cables were placed across the Caban Coch reservoir to prevent enemy planes from landing there – possibly in retaliation.


Claerwen Reservoir

As part of the original 1892 plans for the Elan Valley dams were for three dams in the associated Claerwen Valley. These would be built once the demand for water justified them. Plans were delayed by the First World War. Ironically, once there was a possibility of the dams being built, this was delayed by the Second World War. As well as delaying the extension of the system, the existing reservoirs were put under strain because of wartime production.

So when the Second World War ended plans were finally put into operation. However advances in civil engineering and construction meant larger dams could now be built - meaning that only one dam need be built rather than the originally proposed three.

Apart from size, construction of this dam differed from the earlier ones in several ways. This was concrete - although it was finished in stone. Being the late 1940s, there was a demand on British stonemasons on rebuilds following the war - in particular the Houses of Parliament. So over 100 Italian Stonemasons were drafted to work on the dam.

Commencement of work no longer depended on a railway - all material was brought to the site by road transport (although much was taken as far as Rhayader by rail). Automation also meant that large numbers of manual labourers were no longer needed, 470 men were employed on the construction.

The Claerwen is the most massive reservoir in the complex - its capacity being almost that of all the other reservoirs put together.

One of the first official engagements of the reign of Queen Elizabeth II was to open the Claerwen Dam in October 1952.


The Unfinished Reservoir

When the Claerwen Reservoir was envisaged as being three dams, the lowest would have had a base lower than the maximum height of the Caban Coch reservoir. This meant that its base up to the height of the Caban Coch had to be built before the earlier reservoir was completed.

The base of this unfinished reservoir - the Dol-y-Mynach dam - can be seen today when levels of the Caban Coch reservoir are low. It shows the massive rocks that are used in filling the earlier dams.

If completed the Dol-y-Mynach dam would have been 286 metres (938 ft) wide and 31 metres (101 ft) high.


A Year on the Wye DVD

A Year on the Wye DVD cover


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The river Elan flows from the Mid Wales WildernessThe river Elan flows from the Mid Wales Wilderness
The river Elan flows from the Mid Wales Wilderness © Star Video 2011


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content of external web sites.







The River ElanThe River Elan
The river Elan © Star Video 2011
The Penygarreg DamThe Penygarreg Dam
The Penygarreg Dam © Star Video 2011







The Claerwen ReservoirThe Claerwen Reservoir
The Claerwen Reservoir © Star Video 2011













The Craig Goch ReservoirThe Craig Goch Reservoir
The Craig Goch Reservoir © Star Video 2011




The Penygarreg ReservoirThe Penygarreg Reservoir
The Penygarreg Reservoir © Star Video 2011


The Carreg Ddu ReservoirThe Carreg Ddu Reservoir
The Carreg Ddu Reservoir © Star Video 2011
The Carreg Ddu ReservoirThe Carreg Ddu Reservoir
The Carreg Ddu Reservoir, with the water level low © Star Video 2011
The New Baptist ChapelThe New Baptist Chapel
The new Baptist Chapel built to replace the one that was submerged © Star Video 2011
The Foel TowerThe Foel Tower
The Foel Tower © Star Video 2011




The Caban Coch dam - full flowThe Caban Coch dam - full flow
The Caban Coch dam - water in full flow © Star Video 2011



The Nant-y-Gro DamThe Nant-y-Gro Dam
The Nant-y-Gro Dam – breached in tests that led to the Dambuster Raids © Star Video 2011
The Claerwen DamThe Claerwen Dam
The massive Claerwen Dam © Star Video 2011










The unfinished Dol-y-Mynach DamThe unfinished Dol-y-Mynach Dam
The unfinished Dol-y-Mynach Dam © Star Video 2011



© 2011 Star Video