Rising at about 650m between the summits of Fan Gyhirych and Fan Fraith, the river turns south to flow over Old Red Sandstone rocks before disappearing into the ground when it reaches the band of Carboniferous Limestone.  It shortly reappears and tumbles over a series of cascades and waterfalls formed by resistant bands of sandstone within the Millstone Grit Series of rocks.  Foremost amongst these is perhaps Scwd Ddwli downstream of Pont Melin-fach.  

The river merges with the Afon Hepste at Pontneddfechan to form the Afon Nedd or River Neath.

A former tramway running beside the river from behind the Angel Inn at Pontneddfechan gives access to its lower reaches and to its major west bank tributary, the Afon Pyrddin.  The tramway once served a series of silica mines in the gorge.  The adits (horizontal tunnels) can still be seen though should not be entered.