The river rises as Blaen Senni to the north of the peak of Fan Nedd and east of Fan Gyhirych (), occupying a deep glacial trough until it reaches the hamlet of Heol Senni. Before reaching Sennybridge (), it flows past the village of Defynnog, where it is joined on its left bank by its sole significant tributary, the Nant Treweryn (or Treweren).