The headwaters of the Tawe - Blaentawe - rise at around 550m/1850ft to the east of Fan Brycheiniog in the Black Mountain. Joined by the waters flowing from Llyn y Fan Fawr the growing Tawe cascades over southerly dipping slabs of Old Red Sandstone into Glyntawe.  Here it is joined by the Afon Haffes and the waters emerging from the Dan yr Ogof cave systems beneath the Black Mountain.

The geological weakness known as the Cribarth Disturbance crosses the river near Craig-y-nos Country Park and the tightly faulted and folded Carboniferous Limestone here throws up steep buttresses either side of the river; Cribarth and Craig-y-nos to the west and Craig-y-Rhiwarth to the east.

Travelling downstream, the waters of the Byfre Fechan are discharged into the Tawe at Rhongyr Uchaf, having passed through Britain's deepest cave system - Ogof Ffynnon Ddu.

Passing by Pen-y-cae and Ynyswen, Abercraf and Cae'r-Lan, it leaves the Geopark just north of Ystradgynlais.  Its lower course takes it down the Swansea valley to the sea at Swansea Bay.

Key tributaries of the Tawe include the Afon Giedd and Nant Llech, the latter having tumbled over Henrhyd Falls earlier in its course.  At 30m/90ft it is the highest waterfall in Fforest Fawr Geopark.