The River Clydach is a short, steep and fast-flowing river in Monmouthshire

Monmouthshire

Monmouthshire is a county in south east Wales. The name derives from the historic county of Monmouthshire which covered a much larger area. The largest town is Abergavenny. There are many castles in Monmouthshire .-Historic county:...

 and the county borough of Blaenau Gwent

Blaenau Gwent

Blaenau Gwent is a county borough in South Wales, sharing its name with a parliamentary constituency. It borders the unitary authority areas of Monmouthshire and Torfaen to the east, Caerphilly to the west and Powys to the north. Its main towns are Abertillery, Brynmawr, Ebbw Vale and...

 in south 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...

. It lies within the Brecon Beacons National Park. It is around 10 kilometres (6.2 mi) in length.

The river rises on the southern slopes of Mynydd Llangatwg

Mynydd Llangatwg

Mynydd Llangatwg or Llangattock Mountain is a hill in the Brecon Beacons National Park in the county of Powys, south Wales. It is named from the village of Llangatwg which sits in the valley of the River Usk to the north of it...

 (51.8122°N 3.1939°W) then heads south-east through Clydach Dingle past Brynmawr

Brynmawr

Brynmawr is a market town in Blaenau Gwent, south Wales. The town, sometimes cited as the highest town in Wales, is situated at 1,250 to 1,500 feet above sea level and nestled at the head of the South Wales Valleys...

. It then enters the spectacular Clydach Gorge

Clydach Gorge

The Clydach Gorge is a steep-sided valley in south-east Wales down which the River Clydach flows to the River Usk. It runs for 5.6km from the vicinity of Brynmawr in Blaenau Gwent eastwards and northeastwards to Gilwern in Monmouthshire...

, dropping about 300 metres (984.3 ft) to Gilwern

Gilwern

Gilwern is a village in Monmouthshire, Wales. The name is translated from the Welsh language as "the nook near the alder grove". It is within the Llanelly parish ward of Monmouthshire County Council, west of market town of Abergavenny.-Attractions:...

 and its 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...

 

"Clydach" is a common name for watercourses in south Wales and is thought to derive from an old Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

word for "swift" or possibly "stoney", both of which would apply in this case.