The Taf Fechan rises immediately to the south of Pen y Fan and runs southwards through the two Neuadd Reservoirs, Pentwyn Reservoir and Pontsticill Reservoir before descending into a wooded gorge known as the Blue Pool and flowing south-westwards around the hamlet of Trefechan. To the south of Cefn-coed-y-cymmer, the Taf Fechan and Taf Fawr merge at the wooded confluence that gives Cefn-coed-y-cymmer its name. The river holds a large population of wild brown trout, many of which were killed in 2006 when a large quantity of aluminium sulphate was flushed into the river from the Pontsticill Water Treatment Works, which United Utilities operate on behalf of Dwr Cymru Welsh Water. The pollution turned the water white for at least 5 km downstream of the works and killed approximately 23,000 fish, including wild brown trout, bullheads, stickleback and pike, and a large number of invertebrates. Environment Agency Wales estimated that the river would take six to twelve years to recover. The trout population had resumed breeding by 2009.
Tributaries
Nant Ffrwd
Underneath the Cefn Coed Viaduct, the Nant Ffrwd, a tributary that runs eastward in a deep wooded gorge before flowing into the Taf Fawr above the confluence with the Taf Fechan, emerges into the river from a deep, narrow gorge.
The Taff continues south through the centre of Merthyr Tydfil, where it is joined by the Nant Morlais which emerges at Abermorlais from a culvert in the east bank. Just south of Merthyr weir, the Nant Rhydycar joins.
South of Merthyr, the Taff begins to meander its way between Pentrebach and Abercanaid and through Troed-y-rhiw, Merthyr Vale and Aberfan towards Quakers Yard. Here, the Taff Bargoed, Nant Mafon and then down into Abercynon in the Cynon Valley where the Afon Cynon merge with the Taff and, slightly further downstream the Nant Clydach joins. At Pontypridd it is joined by the Afon Rhondda. From Pontypridd, it runs roughly south, through Taff's Well and Radyr, before arriving at the northern boundary of the city of Cardiff in Llandaff.
Mouth of the Taff
In Cardiff, the natural course of the river was changed during the 19th century; from Cardiff Castle it now follows an artificial riverbed west of where it previously flowed, following a path through Bute Park, past the Cardiff Arms Park and the Principality Stadium, and into Cardiff Bay, an artificial lake due to the construction of a barrage, to flow into the Severn Estuary.
"Taffy" as a pejorative
The derogatory term "Taffy" for a Welsh person is sometimes erroneously stated to have originated with the river. This may have given rise to a colloquial usage of the term by Cardiff residents for people from the Taff's northern reaches. The name actually originates as a corruption of the name Dafydd, with the Oxford English Dictionary describing the origin as "representing a supposed Welsh pronunciation of the given name Davy or David ". It was common for people in times of war to dehumanise an enemy by ascribing a singular name to them all. It is therefore an equivalent of other historic English pejoratives such as Paddy and Jock. The term is a common nickname within the British army, with individual Welsh troops, units and especially the Welsh Guards sometimes referred to as "The Taffs". However, it has been reported that the guardsmen themselves never use that term.