The Afon Hydfer is a short river in Powys

Powys

Powys is a local-government county and preserved county in Wales.-Geography:Powys covers the historic counties of Montgomeryshire and Radnorshire, most of Brecknockshire , and a small part of Denbighshire — an area of 5,179 km², making it the largest county in Wales by land area.It is...

 in the west of the Brecon Beacons National Park, south Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. Its headwaters rise on the northeastern slopes of the Black Mountain

Black Mountain (range)

The Black Mountain is a mountain range in Mid and West Wales, straddling the county boundary between Carmarthenshire and Powys and forming the westernmost range of the Brecon Beacons National Park. Its highest point is Fan Brycheiniog at 802 metres or 2,631 ft. The Black Mountain also forms a part...

 as the streams named Trinant, Nant y Cadno, Nant yr Erydr and Crognant joining to flow down Cwm yr Afon on the western edge of Glasfynyd Forest. The river turns east at Blaenau Uchaf, then northeast through Cwm Hydfer and joins the River Usk

River Usk

The River Usk rises on the northern slopes of the Black Mountain of mid-Wales, in the easternmost part of the Brecon Beacons National Park. Initially it flows north into Usk Reservoir, then east by Sennybridge to Brecon before turning southeast to flow by Talybont-on-Usk, Crickhowell and...

just below Pont ar Hydfer. Other tributaries of the Hydfer include the streams of Nant Cil-y-clawdd, Nant Crinog and Nant Meity.