Afon Twrch
It forms the boundary between the counties of Brecknockshire ( currently administered as part of the unitary authority of Powys ) and Carmarthenshire and , downstream of Ystradowen , between Brecknockshire and Glamorgan ( and unitary authorities of Powys and Neath Port Talbot ) . 
<triplet> Brecknockshire <subj> Powys <obj> located in the administrative territorial entity <triplet> Powys <subj> Carmarthenshire <obj> shares border with <subj> Neath Port Talbot <obj> shares border with <triplet> Carmarthenshire <subj> Brecknockshire <obj> shares border with <subj> Powys <obj> shares border with <subj> Neath Port Talbot <obj> shares border with <triplet> Neath Port Talbot <subj> Powys <obj> shares border with <subj> Carmarthenshire <obj> shares border with