The source of the Afon Sawdde is Llyn y Fan Fach, the body of water associated with the legend of the 'lady of the lake'.  Its waters tumble steeply down 250m/800ft into Cwmsawdde on their way to meet those of the River Towy/Afon Tywi at Llangadog.

Where the river cuts through a series of tough Silurian and Devonian sandstones it has formed a narrow gorge, best developed for the 0.5miles/1km downstream from Pont-ar-llechau beside the A4069.  This section of valley with its rocky river bed has been declared a site of special scientific interest (SSSI) for its almost complete - and certainly well exposed - section through the Silurian rocks of the region.