The river rises in the mountains south of Machynlleth, reaches the hill walk of Uwch Garreg and then plunges over a series of waterfalls known as Pistyll y Llyn, into a ravine, descending towards the River Dyfi (or Dovey) near Dovey Junction railway station.

It flows northwards through mountainous scenery for about three miles,[1] before suddenly coming to the edge of the escarpment and plunging into a ravine in a series of waterfalls and cascades known as Pistyll y Llyn, descending abruptly from 370 to 210 metres (1,210 to 690 ft).

[2] At a more leisurely pace, it now begins to curve gradually towards the west and passes the hamlet of Gellifudr at the end of the first unclassified road it has encountered.

The road is now on its left bank as it continues westwards through a steep-sided valley with woodland on either side.

Reaching more level ground, it flows under the A487 road at Pont Llyfnant and under the Cambrian Coast Line from Shrewsbury to Aberystwyth close to Dovey Junction railway station, before joining the River Dovey about one mile inland from its estuary.