The Clywedog reservoir is a reservoir near Llanidloes, Wales. Completed in 1967, the reservoir was built near the B4518 road, Powys, to supply water to Birmingham and the English Midlands by river regulation and to mitigate flooding in the lower Severn by storing excess winter rainfall. The reservoir was formed by damming the Afon Clywedog, a tributary of the River Severn. 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. A second, much smaller embankment dam is located at Bwlch-y-gle to prevent overflow into the next valley.