[1] The water emerging at Llygad Cynon has been traced back to the sink of the Nant Cadlan at Ogof Fawr.

[2] The Nant Cadlan and its tributary the Ceunant Du rise on the slopes of Cefn Cadlan to the northeast of Penderyn flowing initially southeast and then south on the east side of the village to the point where the Cynon arises, adding to the flow emerging from the rising.

The final tributary of any size is the Nant Pennar which enters on the Cynon's left bank at Mountain Ash (Welsh: Aberpennar).

[5] While Thomas Morgan stated that the name was a compound of the elements "cyn" (meaning chief) and "ain" (water or brook), giving a meaning of the "chief brook",[6] modern writers more commonly state that the river derives from a personal name, Cynan or Cynon.

[8] Together with two neighbouring rivers, it gives its name to a local authority, with unitary status, Rhondda Cynon Taf.