The Afon Clywedog is a tributary of the River Severn approximately 29 km long which has its source on the flanks of Plynlimon and has its confluence with the Severn in Llanidloes. The annual rainfall over the catchment is between 1737 mm and 1836 mm. In 1979 the 90 low was 5.266 m3/s and the 5 low was 0.292 m3/s.