The River Dulais rises below the slopes of Mynydd y Drum in the foothills of the Brecon Beacons.
The waterway flows down the Dulais valley for approximately 13 kilometres (8 miles), south-west through the villages of Seven Sisters and Crynant before cascading over the Aberdulais Falls. Here it joins the River Neath close to the tidal reaches near Tonna.
An ice age beginning
The gorge in which the river and waterfall now lie was formed about 20,000 years ago. As a glacier further up the valley melted, the resulting melt water slowly cut its way down through the 300-million-year-old rock. This can be seen on the west side of the gorge today.
The rock is Pennant sandstone, which is a very severely compressed bed of sand. Beneath it is a layer of coal, that has been gradually eroded by the flowing water allowing the rock above to collapse and form the Falls as we see them today. Originally the Falls were further south, but over the centuries, with continued erosion, they've been slowly cut back to their present location.
A wet weather spectacle
The Dulais is a flash flood river, which means it rises and falls very quickly. In wet weather, and in winter, when the river is in full spate, it's a truly awesome - and noisy - spectacle.