|Basin countries||United Kingdom|
|Surface area||3.688 km²|
The Claerwen reservoir and dam in Powys, Wales, were the last additions to the Elan Valley Reservoirs system built to provide water for the increasingly demanding city of Birmingham, in neighbouring England. Built mainly of concrete, the exterior of the dam face is dressed stone. The dam itself is of a type called a gravity dam. This type of dam needs to be built upon solid foundations as the pressure of the reservoir behind should be in equilibrium with the total weight of the dam itself thus causing complete stability.
The Claerwen dam was finished in 1952 and was given a late Victorian effect so that it blended in with the earlier dams in the valley. It was necessary to employ the services of Italian stonemasons as British ones were still at work in London during the post-war rebuilding process of the late 1940s.
The dam took six years to complete and was almost twice the size of the other dams in the Elan valley. The Claerwen reservoir is almost the size of all the other reservoirs in the Elan Valley system combined. Officially commissioned by Queen Elizabeth II in 1952, it was one of her first royal engagements.
- Height: 56 m (184 ft)
- Length: 355 m (1,165 ft)
- Building material: primarily concrete
- Capacity: 48,300,300 m3 (1.06246×1010 imp gal)
Claerwen NNR 
Claerwen National Nature Reserve is a lonely expanse of mountain upland lying half way between Rhayader and Pontrhydfendigaid in Powys. Although its landscape can seem bleak and inhospitable, the mainly peaty and acidic soil provides a home for numerous species of plants and animals which thrive in these conditions. One of the three British species of carnivorous sundew plants or Drosera can be found here, and it is a known breeding site for the rare Merlin and Red Kite birds.
- Photos of Claerwen reservoir and surrounding area on geograph.org.uk
- Photos of Claerwen NNR and surrounding area on geograph
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