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Clywedog Valley
Historic Landscape
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Historic Landscape Characterisation

The Clywedog Valley: Clywedog Reservoir
Trefeglwys and Llanidloes Without communities, Powys
(HLCA 1190)


Concrete dam and reservoir constructed during the 1960s in the steep-sided upper Clywedog valley to regulate water supplies in the Severn valley.

Historic background

The area fell within the manorial township of Esgeiriaeth in the Montgomeryshire tithe parish of Trefeglwys and the manorial townships of Ystradhynod and Brithdir in the 19th-century Montgomeryshire tithe parish of Llanidloes.

Key historic landscape characteristics

Sinuous modern reservoir at a height of about 290 metres above sea level, covering an area of about 240 hectares. Prior to flooding the valley floor would have been covered with slowly permeable and seasonally waterlogged fine silty and clayey soils overlying drift deposits derived from mudstones and shales, economically best suited to stock rearing and dairying on permanent grassland.

Following an enabling Act of Parliament, the dam and reservoir were constructed between 1964 and 1967 by damming the river Clywedog. The dam is 72 metres high and 230 metres long and is the tallest concrete dam in the United Kingdom. A second, much smaller embankment dam at Bwlch-y-gle, crossed by the Llanidloes to Machynlleth road (B4518) was built to prevent overflow into the Cerist valley. The reservoir and its dams were designed by the civil engineers Sir William Halcrow & Partners. The main contractors were Reed and Mallick of Salisbury. The dam is constructed of 11 independent concrete buttresses, the areas between the buttresses being hollow internally. It is unusual in curving downstream rather than upstream but its design was the result of concerns about the load-bearing properties of the rocks in the valley sides; rather than the pressure of the full weight of water being thrown onto the sides of the dam, which would have been the case if a single structure with an upstream curve had been constructed, the buttresses are designed to direct the thrust downwards to the valley floor. Construction involved a workforce of about 500 men who were accommodated in temporary buildings on the hillside above the dam near the present viewing area.

The reservoir involved the flooding of much of the former agricultural land represented by small irregular fields and a number of farms and former farms and cottages, including those at Aber-biga, Gronwen, Eldid, Croes-isaf, Grodir, Coppice-llwyd (Cwm-pwll-llwyd) and Llwybr-y-madyn, Ystrad-hynod, Merllyn and Draws-y-nant, many of which were probably of medieval to early post-medieval origin. Other archaeological sites affected included the Ystradhynod Bronze Age burial mound and standing stone, excavated in 1965-66 in advance of flooding, which lay on the valley floor below Dinas, sited relatively close to the banks of the river Clywedog. In addition to some local opposition, similar to that which had accompanied the construction of the Tryweryn reservoir near Bala a few years earlier, construction work was delayed for several months in 1966 by a bomb detonated within the construction site, for which it was widely suspected that a political extremist group, Mudiad Amddiffyn Cymru (MAC), was responsible.

The reservoir generally fills with water over the winter months and at full capacity the reservoir holds about 50,000 megalitres of water. The prime purpose of the reservoir is to enable public water supply abstractions from the entire length of the river Severn to be sustained during dry summer months, whilst ensuring sufficient flow in the river to sustain environmental needs, though it also plays some role in flood prevention, particularly in the upper reaches of the Severn. The reservoir is currently owned and operated by Severn Trent Water Limited with oversight and regulation from the Environment Agency. The dam operating plant runs self-sufficiently via the use of a 500 kW hydroelectric turbine. Llyn Clywedog is now a popular leisure amenity, offering a range of recreational activities including walking, cycling, bird watching, angling, windsurfing and sailing.


Historic Environment Record; Cadw Listed Building descriptions; modern Ordnance Survey 1:10,000, 1:25,000 mapping and 1st edn Ordnance Survey 1:2,500 mapping; ApSimon 1973; Hamer 1872; RCAHM 1911; Krause 1983; Richards 1969; Soil Survey of England and Wales

For further information please contact the Clwyd-Powys Archaeological Trust at this address, or link to the Countryside Council for Wales' web site at

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