|Primary inflows||Afon Clywedog|
|Primary outflows||Afon Clywedog|
|Max. length||9.5 km (5.9 mi)|
|Surface area||2.5 km2 (0.97 sq mi)|
|Max. depth||66 m (217 ft)|
|Water volume||50,000 megalitres (41,000 acre⋅ft)|
The Clywedog reservoir (Welsh: Llyn Clywedog) is a reservoir near Llanidloes, Wales on the head-waters of the River Severn. The construction of the reservoir was enabled by an Act of parliament which asserted that "The chief function of the reservoir is river regulation  It was completed in 1967 and is situated near the B4518 road north of Llanidloes, Powys   Its function is to regulate the flow in the River Severn by releasing water into the river channel during low flow periods and re-filling during the wetter winter months. Capacity is held in the reservoir thoughout the winter so that it retains capacity to mitigate downstream flooding by absorbing excess flow from the head-waters of the Afon Clywedog, a tributary of the River Severn . The reservoir was formed by damming both the Afon Clywedog and a much smaller embankment dam located at Bwlch-y-gle to prevent overflow into the next valley. Its concrete buttress dam is the tallest concrete dam in the UK, with a height of 72 metres and a length of 230 metres. When at capacity the reservoir contains approximately 50,000 megalitres of water. .
Construction of the dam started 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 (2.5 km2) 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 (MAC) 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 Powys County Councils 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.
- Keith Smith (1972). "Water in Britain: A Study in Applied Hydrology and Resource Geography". ISBN 978-1-34901499-6.
- "CLYWEDOG DAM, LLANIDLOES". Coflein. Retrieved 17 March 2018.
- "Llanidloes - Llyn Clywedog". Llanidloes Town Council. Retrieved 17 March 2018.
- "Our history – Clywedog Dam, Wales − 1967". Halcrow Group website. Halcrow Group Ltd. 2011. Archived from the original on 30 December 2011. Retrieved 8 January 2012.
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