A significant tributary of the Tywi on the Geopark’s northern edge.
The river begins life as the Nant Gwydderig on the south-western slopes of Mynydd Epynt. It turns sharply westwards at Llywel and soon enters a deep valley followe by the A40 road. This wooded gorge, which divides the high ground of the Epynt from that of the Black Mountain, was probably cut by glacial meltwater during the ice age.
The Gwydderig joins with the Afon Bran at Llandovery and their combined waters enter the Tywi shortly thereafter.