The Clywedog reservoir (Welsh: Llyn Clywedog) 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.