The Afon Clun is a 14-mile (23 km) long tributary of the River Ely (Welsh: Afon Elai), in the counties of Cardiff and Rhondda Cynon Taf, Wales. Its bedrock is predominantly of sandstone. Beginning on the western slope of The Garth (Mynydd y Garth) the river is fast-flowing, in clear shallow water with a hard substrate, flowing to the south of Llantrisant and generally west to its confluence with the River Ely at Pontyclun, falling 715 feet (218 m) over its course.