It rises on the eastern slopes of Fan Gyhirych in the Fforest Fawr section of the Brecon Beacons National Park and flows south for 7 miles (11 km) to join with the Afon Mellte at Pontneddfechan, their combined waters continuing as the River Neath (Welsh Afon Nedd) to the sea near Swansea.

Downstream of this confluence it forms the boundary between Powys to its east and Neath Port Talbot to its west.

[1] The headwaters of the river run over Old Red Sandstone as far as Blaen-nedd-isaf where they encounter the Carboniferous Limestone outcrop.

Each of the falls is associated with faults bringing harder sandstone beds into contact with the more readily eroded mudstones.

It also forms a part of a larger special area of conservation designated for its rare bryophyte flora.