The river continues, passing the abandoned workings of the Cwm Rheidol lead mine; one of many other metal mines in the valley – a source of extracted metal pollution of the river – and flows westwards before reaching its confluence with the Afon Ystwyth and the estuary at Aberystwyth to drain into Cardigan Bay.

The traditional source is Llyn Llygad Rheidol, a lake in a high valley near the summit of Pen Pumlumon Fawr, as its name suggests ("Rheidol's Eye Lake"); the stream Nant y Llyn descends from it into the Afon Hengwm, but is not a substantial stream.

There are several other, larger streams, such as the Afon Hengwm, which rises to the south of Llyn Bugeilyn, just across the boundary in Powys.

This was originally built to ship metal ore from the mines, but now provides a very popular tourist route to the top end of the Rheidol valley.

A large stag-like shape on the northern valley wall, above the Cwm Rheidol reservoir, known as the "White Stag", is traditionally said to be a hill figure but it is in fact an industrial feature, formed by the waste from lead mining at Gellireirin.

Within the valleys, dense and ancient oak forests with rich understoreys of ferns, mosses and lichens are common.