The Clywedog Reservoir () 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 "At certain times the flow of water in the river is inadequate ... unless that flow were regulated so as to ensure that at those times water in addition to the natural flow will flow down the river." Purpose 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 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 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 and a length of . When at capacity the reservoir contains approximately 50,000 megalitres of water. Construction 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 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. Operation 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