The Twrch rises high on the Black Mountain.  In fact its headwater streams reach to around 2500ft / 760m on the western slopes of Fan Brycheiniog.  Some of these streams have been captured by the headwaters of the Afon Sychlwch in relatively recent times, their waters now flowing into Llyn y Fan Fach then on down the Sawdde to the Towy instead.

The Afon Twrch forms the ancient boundary between Carmarthenshire and Brecknockshire (now Powys) from Fan Foel down to Ystradowen.

The name (English: 'boar river') may originate from the supposed 'snouting' action of its waters in their steep descent from the heights of the Black Mountain to their confluence with the Afon Tawe at Ystalyfera.