Afon Senni

The Afon Senni is a river in Powys, Wales which rises in the Fforest Fawr section of the Brecon Beacons National Park and flows northwards for 11 kilometres (7 mi) to a confluence with the River Usk at Sennybridge.

The river rises as Blaen Senni to the north of the peak of Fan Nedd and east of Fan Gyhirych (51°51′44″N 3°35′35″W / 

), occupying a deep glacial trough until it reaches the hamlet of Heol Senni. Before reaching Sennybridge (51°56′44″N 3°34′20″W / 51.9455°N 3.5723°W / 51.9455; -3.5723 (Afon Senni (mouth))), 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).^[1]

The river's name probably derives from a personal name 'Senni'.^[2]

The river is designated as a special area of conservation in respect of its population of three species of lamprey, twaite shad, European bullhead, Atlantic salmon and otter.^[3]