The Rheidol rises in the headwaters of the Nant-y-moch Reservoir () on the western flanks of Plynlimon, near the sources of the Wye and Severn. After flowing south to Ponterwyd on the increasingly deep valley, then southwest through Welsh Oak ancient woodland, it veers westwards to its confluence with the Afon Mynach, at Devil's Bridge, Ceredigion (, lit. The bridge on the Mynach) and the most spectacular of waterfalls. 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 its confluence with the Afon Ystwyth and the estuary at Aberystwyth to drain into Cardigan Bay. ()