Course
The Afon Clun, a major tributary of the River Ely, drains an area of 12 square miles (31 km2) to the north-west of Cardiff in south Wales. The river's source is on the western slope of The Garth (Welsh: Mynydd y Garth) (elevation 1,007 feet (307 m)), a mountain of pennant sandstone midway between Cardiff (Welsh: Caerdydd) and Pontypridd, whose eastern slopes feed the River Taf by Taff's Well (Welsh: Ffynnon Taf). The Garth stands in the north western corner of the City and County of Cardiff (Welsh: Dinas a Sir Caerdydd), above the village of Gwaelod y Garth, about seven miles (11 km) north west of Cardiff city centre and a mile (1.6 km) north of Pentyrch. The lower northern slopes of The Garth form the boundary with Rhondda Cynon Taf, about half a mile (800 m) north of the Clun's source. To the east the land falls away sharply from the summit, dropping more than 650 feet (200 m) over a distance of 1,200 feet (366 m). The drop is less dramatic on the western slopes, the source of the Clun, but the river still falls to 165 feet (50 m) by the time it crosses the county boundary into Rhondda Cynon Taf at Rhiwsaeson, just over two miles (3.5 km) from its source. The Clun is fast-flowing, with clear shallow water and a hard substrate (gravel/cobble/pebble).
Near the Dŵr Cymru / Welsh Water water treatment plant at Rhiwsaeson the Afon Clun is fed from the north by Nant Myddlyn, which itself is joined from the east by Nant Dowlais. Nant Myddlyn rises in Tynant, Beddau about one and a half miles (2.5 km) to the north of its confluence with the River Clun, between Beddau and Llantwit Fardre (Welsh: Llanilltud Faerdref), and Nant Dowlais rises in Church Village (Welsh: Pentre'r Eglwys). Nant Dowlais also has a tributary, which rises on Garth Isaf, two and a quarter miles (3.5 km) from it on the north western slopes of The Garth and about half a mile (800 m) north of the Clun's source.
Leaving Rhiwsaeson, about a mile (1.5 km) east of Cross Inn, the Clun widens to between about eight (2.5 m) and 12 feet (3.5 m) and slows. Here, where the Clun flows to the south of Cross Inn, as well as along the banks of Nant Dowlais and Nant Myddlyn, evidence of otter activity has been noted. The river flows to the south of the A473 Pontypridd to Bridgend (Welsh: Penybont ar Ogwr) road (Talbot Green By-Pass), where it is fed from the south by Nant Mwyndy. Nant Mwyndy flows over a bedrock of Carboniferous Limestone, rising in Creigiau and, flowing westwards, immediately north of Groesfaen, passing a small Industrial Estate at Mwyndy, feeding a lake resulting from Victorian opencast iron mining (see History below) and turning south to flow past Cefn-y-Parc Cemetery (Welsh: Mynwent Cefn-y-Parc), Penygawsi, before reaching its confluence with the Afon Clun.
The Clun then flows beneath the main A4119 (Tonypandy to Cardiff Bay) route about 50 metres (164 ft) south of the roundabout by Glamorgan Vale Retail Park, Talbot Green. At this point, the river often overflows onto the meadows to the north, providing a wetland wildlife habitat, although drainage of floodplain grasslands, for industrialisation, housing and associated infrastructure has had an impact on wildlife by reducing its available area. To the south, the enclosed woodland of Coed-yr-Hendy follows the river's course for its final half a mile (800 m). The woodland's gentle slope, up from the Clun towards Miskin (Welsh: Meisgyn) and Pontyclun, prevents significant flooding over Afon Clun's southern bank.
After passing Y Pant Comprehensive School and Pontyclun Fire Station, the Clun flows under the bridge built for the Llantrisant to Cowbridge (Welsh: Y Bont Faen) main road, also known as the A4222, and gives its name to the nearby village, Pontyclun, which translates into English as 'the (river) Clun bridge'. Immediately past the bridge, 14 miles (23 km) from its source, is the Afon Clun's confluence with the River Ely, which heads south, east to Miskin, almost encircling Pontyclun, then south on its way to Cardiff, where it flows into Cardiff Bay by Penarth Marina, which flows into the Bristol Channel (Welsh: Môr Hafren).
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