|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". Its primary purpose was to regulate the flow in the River Severn to mitigate flooding and provide support for drinking water abstractions for the West Midlands. It was completed in 1967 and is situated near the B4518 road north of Llanidloes, Powys.
It also regulates the flow of water in the River Severn by releasing water into the river channel during low flow periods and re-filling during the wetter winter months. This enables major water abstractions to be made from the River Severn to supply the West Midlands with 50 million litres (11,000,000 imp gal) of drinking water. Capacity is held in the reservoir throughout 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 (236 ft) and a length of 230 metres (750 ft). 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 two months. The political extremist group Mudiad Amddiffyn Cymru (MAC) was widely suspected of carrying out the bombing.
The reservoir was opened in 1967 and has been in continuous usage since then, 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 Council's 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. At the base of the dam lies the ruins of the Bryntail lead mine. In 2005 Natural Resources Wales installed an osprey nest platform near the reservoir. The nest has been in use by breeding ospreys since 2014; by the end of 2019, 15 osprey chicks had been raised on the site. A web camera was installed in 2020 enabling video of the nest to be viewed online.
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- "Osprey webcam set up in Hafren Forest near Llanidloes". Powys County Times. Retrieved 12 June 2020.
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