The Afon Senni is a river in Powys

Powys

Powys is a local-government county and preserved county in Wales.-Geography:Powys covers the historic counties of Montgomeryshire and Radnorshire, most of Brecknockshire , and a small part of Denbighshire — an area of 5,179 km², making it the largest county in Wales by land area.It is...

, Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 which rises in the Fforest Fawr

Fforest Fawr

Fforest Fawr is the name given to an extensive upland area in the county of Powys, Wales. Formerly known as the 'Great Forest of Brecknock' in English, it was a royal hunting area for several centuries but is now used primarily for sheep grazing, forestry, water catchment and recreation...

 section of the Brecon Beacons National Park and flows northwards for 11 kilometres (6.8 mi) to a confluence with the River Usk

River Usk

The River Usk rises on the northern slopes of the Black Mountain of mid-Wales, in the easternmost part of the Brecon Beacons National Park. Initially it flows north into Usk Reservoir, then east by Sennybridge to Brecon before turning southeast to flow by Talybont-on-Usk, Crickhowell and...

 at Sennybridge

Sennybridge

Sennybridge is a village in Powys, Mid Wales, situated some from Cardiff and from Swansea. It lies west of Brecon on the A40 trunk road to Llandovery, at the point where the River Senni flows into the Usk...

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The river rises as Blaen Senni to the north of the peak of Fan Nedd

Fan Nedd

Fan Nedd is a mountain in the Fforest Fawr area of the Brecon Beacons National Park in Wales. In common with the rest of the Fforest Fawr uplands it is within the county of Powys....

 and east of Fan Gyhirych

Fan Gyhirych

Fan Gyhirych is a mountain in the Fforest Fawr section of Brecon Beacons National Park in south Wales. It lies within the county of Powys, formerly Brecknockshire....

  occupying a deep glacial trough until it reaches the hamlet of Heol Senni

Heol Senni

Heol Senni is a hamlet in the valley of the Afon Senni just north of the Fforest Fawr section of the Brecon Beacons National Park. It lies within the community of Maescar in the county of Powys, Wales. The Welsh name means the 'road by Senni' and reflects its position near the crossing of the...

. Before reaching Sennybridge it flows past the village of Defynnog

Defynnog

Defynnog is a small village in the community of Maescar in the county of Powys, Wales. It lies immediately south of Sennybridge and a few miles west of Brecon within the Brecon Beacons National Park...

, where it is joined on its left bank by its sole significant tributary, the Nant Treweryn (or Treweren).

The river name probably derives from a personal name 'Senni'.

The river is designated as a special area of conservation

Special Area of Conservation

A Special Area of Conservation is defined in the European Union's Habitats Directive , also known as the Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora...

 in respect of its population of three species of lamprey

Lamprey

Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

, twaite shad, European bullhead

European bullhead

The bullhead is a freshwater fish that is widely distributed in Europe, mainly in rivers. It is a member of the Cottidae family, a type of sculpin...

, Atlantic salmon

Atlantic salmon

The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....

 and otter

Otter

The Otters are twelve species of semi-aquatic mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals....

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