The River Cynon is a river in South Wales

South Wales

South Wales is an area of Wales bordered by England and the Bristol Channel to the east and south, and Mid Wales and West Wales to the north and west. The most densely populated region in the south-west of the United Kingdom, it is home to around 2.1 million people and includes the capital city of...

 and is one of the larger tributaries of the River Taff

River Taff

The River Taff is a large river in Wales. It rises as two rivers in the Brecon Beacons — the Taf Fechan and the Taf Fawr — before joining to form the Taff north of Merthyr Tydfil...

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It arises within Cwm Cadlan in the Brecon Beacons National Park as the Nant Cadlan and is supplemented by a number of small streams arising from springs in the Carboniferous Limestone

Carboniferous limestone

Carboniferous Limestone is a term used to describe a variety of different types of limestone occurring widely across Great Britain and Ireland which were deposited during the Dinantian epoch of the Carboniferous period. They were formed between 363 and 325 million years ago...

 and Twrch Sandstone in the vicinity of Penderyn

Penderyn

Penderyn may refer to:*Penderyn, Rhondda Cynon Taf, a village in Wales*Penderyn , a whisky produced by Penderyn Distillery...

. The spring of Llygad Cynon is a notable example. However the young river soon crosses into the Carboniferous

Carboniferous

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 Coal Measures

Coal Measures

The Coal Measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. It represents the remains of fluvio-deltaic sediment, and consists mainly of clastic rocks interstratified with the beds of coal...

 as it passes through Hirwaun

Hirwaun

Hirwaun is the name of a village at the northwest end of the Cynon Valley in the County Borough of Rhondda Cynon Taf, South Wales. The village of Hirwaun is from the town of Aberdare, and comes under Aberdare for postal reasons...

. It is crossed by the A465

A465 road

The A465 is a major road in south Wales. It is more commonly known as the Heads of the Valleys Road because it joins together the north ends of the South Wales Valleys...

 Heads of the Valleys Road near what was until 2008 the last working deep mine in Wales, Tower Colliery

Tower Colliery

Tower Colliery is the oldest continuously working deep-coal mine in the United Kingdom, and possibly the world, and the last mine of its kind to remain in the South Wales Valleys...

. The remainder of the valley is strongly marked by the signs of past industrialisation including colliery waste tips, the relics of a large smokeless fuel

Smokeless fuel

Smokeless fuel means fuel which does not produce visible smoke when burned. The term is usually applied to solid fuels, such as:* Anthracite* Coke* Charcoal* Hexamine fuel tablets...

 plant at Abercwmboi

Abercwmboi

Abercwmboi is a village in the Welsh county borough of Rhondda Cynon Taf in Wales, United Kingdom.- Location :Abercwmboi is one of the last remaining villages in the Cynon Valley. It has retained its identity and not been developed as many other Cynon Valley villages. As a result is a very close...

 which includes the remnants of a tip that burned out of control for many years. The river passes in a south easterly direction through Aberdare

Aberdare

Aberdare is an industrial town in Rhondda Cynon Taf, Wales, situated at the confluence of the Rivers Dare and Cynon. The population at the census was 31,705...

, Mountain Ash

Mountain Ash, Wales

Mountain Ash is a town and community in Rhondda Cynon Taf, deep in the South Wales Valleys of Wales. Mountain Ash is situated in the Cynon Valley and has a population of 7,039...

, Penrhiwceiber

Penrhiwceiber

Penrhiwceiber is a former Welsh coal mining village and community deep in the South Wales Valleys of Wales. Penrhiwceiber is situated in the Cynon Valley in the county borough of Rhondda Cynon Taf, it lies 1 mile south of the town of Mountain Ash...

 before eventually meeting up with the River Taff at Abercynon

Abercynon

Abercynon is a small village in the Cynon Valley in Mid Glamorgan, Wales. The unitary authority is now known as Rhondda Cynon Taff. It is composed of the village of Abercynon itself,Carnetown,Glancynon,Park View and Pontcynon. However, in recent years the sign to show motorists they are entering...

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The quality of the river has improved dramatically following the closure of the coal industry and the demolition of the smokeless fuel plant, but these closures had significant financial and social impacts on the communities in the valley and local priorities may be more focussed on social re-generation than environmental improvements to the river.