The River Dulais rises on Mynydd y Drum in the western Brecon Beacons. It flows down the Dulais Valley, through the villages of Seven Sisters and Crynant, before reaching its confluence with the River Neath after about . Here it flows over a ridge of Pennant sandstone, entering a gorge that was carved by meltwater from a glacier 20,000 years ago. Originally the falls were nearer the sea, but the water has eroded the rock and cut its way back to its present location. The River Dulais is prone to flash floods when heavy rain falls in its catchment area.