TheAfon Senni is a river inPowys,Wales which rises in theFforest Fawr section of theBrecon Beacons National Park and flows northwards for 11 kilometres (7 mi) to a confluence with theRiver Usk atSennybridge.
The river rises as Blaen Senni to the north of the peak ofFan Nedd and east ofFan Gyhirych(51°51′44″N3°35′35″W / 51.8621°N 3.5931°W /51.8621; -3.5931 (Afon Senni (source))), occupying a deepglacial trough until it reaches the hamlet ofHeol Senni. Before reaching Sennybridge(51°56′44″N3°34′20″W / 51.9455°N 3.5723°W /51.9455; -3.5723 (Afon Senni (mouth))), it flows past the village ofDefynnog, where it is joined on its left bank by its sole significant tributary, theNant Treweryn (or Treweren).[1]
The river's name probably derives from a personal name 'Senni'.[2]
The river is designated as aspecial area of conservation in respect of its population of three species oflamprey,twaite shad,European bullhead,Atlantic salmon andotter.[3]
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