Llyn Clywedog

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Llyn Clywedog

Llyn Clywedog or the Clywedog Reservoir is a reservoir or man-made lake in Montgomeryshire, created in the steep valley of the River Clywedog by damming the river at the old lead mines of Pen-y-Gar, some miles above Llanidloes, where the Clywedog meets the River Severn.

Completed in 1967, the reservoir was built to supply water to Birmingham and the Midlands[1] by river regulation and to mitigate flooding in the lower Severn by storing excess winter rainfall. The dam on the River Clywedog is a concrete buttress dam and the tallest concrete dam in the United Kingdom, with a height of 236 feet and a length of 755 feet.

Below Clywedog reservoir dam

When at capacity the reservoir contains approximately 65 million cubic yards of water. A second, much smaller embankment dam is located at Bwlch-y-gle to prevent overflow into the next valley.


Construction of the dam began in 1963 after the passing of an Act of Parliament ordering its creation to help prevent flooding of the River Severn in winter and to maintain its water levels in the summer. Local opposition was strong against the construction of the reservoir as it would result in the flooding of much of the Clywedog valley and the drowning of 615 acres of agricultural land. On top of several disruptions and protests, during construction in 1966 a bomb was detonated within the construction site, setting work back by almost 2 months. The political extremist group Mudiad Amddiffyn Cymru was widely suspected of carrying out the bombing.

The reservoir was opened in 1967 and till this day has been in continuous usage, generally filling with water over the winter months and gradually releasing it during the summer months. The reservoir is currently owned and operated by Severn Trent Water Limited with oversight and regulation by Natural Resources Wales. Clywedog Sailing Club operates on the lake, and the Staylittle Outdoor Centre delivers a range of adventure education both on the water and in the surrounding area.

The dam operating plant runs self-sufficiently from a 500 kW hydro-electric turbine. The area around Clywedog dam is now a popular leisure destination offering scenic walks and wildlife watching.

In a famous television advertisement for the Land Rover Defender, a Defender 90 is seen winching up the face of the dam under its own power.[2]

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