Not to be confused with the River Twrch / Afon Twrch which is a tributary of the Severn rising on the Berwyn range in North Wales or the Afon Twrch which is a tributary of the River Cothi in West Wales.

The Afon Twrch is a river which rises in the Black Mountain in south Wales. It forms the boundary between the counties of Powys (former county of Brecknockshire) and Carmarthenshire and, downstream of Ystradowen, between Powys and Neath Port Talbot county borough.