The river supports several species of migratory fish, including salmon, sewins (sea trout), and eels.

The Taf Fawr rises below the peak of Corn Du, south-west of Pen y Fan and soon flows in a generally southerly direction through a steep-sided valley,[2] which was identified by Cardiff's Borough Engineer John Avery Brandon Williams as the best source of water for the town when he assessed all possible sources of water for the newly formed Cardiff Corporation Waterworks in 1881.

[3] A bill was presented to Parliament in November 1883, and despite serious opposition from riparian landowners, Royal Assent was obtained for the Cardiff Corporation Act 1884 on 7 August.

[6] Construction of Llwyn-on Reservoir, the third in the chain, did not start until 1911, and again, its capacity was increased by an Act obtained in 1909 to 1,260 million imperial gallons (5,700 Ml), almost double that specified in 1884.

[8] Once built, it supplied 7.75 million imperial gallons (35.2 Ml) of compensation water per day to maintain the flow in the river.

It was to have been built of stone, but a masons strike in 1866 resulted in the Brecon and Merthyr Railway buying 800,000 bricks to complete it.

[2] The Taf Fechan rises immediately to the south of Pen y Fan and runs southwards through three reservoirs and a former fourth, now abandoned.

Now decommissioned,[12] the water level when the reservoir was full was at 1,413 feet (431 m) above Ordnance Datum, and it held 74 million imperial gallons (340 Ml).

Although the contractors Holme and King of Liverpool were employed to do much of the preparatory work, the building of the dam was carried out by direct labour, with Merthyr Corporation's surveyor, Thomas F Harvey, acting as resident engineer.

It held 340 million imperial gallons (1,500 Ml), and with a water level at 1,506 feet (459 m), it was one of a very small number of artificial lakes built above the 1,500-foot (457 m) contour.

Repairs undertaken in 1912/13 reduced the level of leakage from 11 to 2 million imperial gallons (50 to 9 Ml) which was sufficient until Pontsticill Reservoir could be built to impound the leaking water.

[18] The corporation had obtained the Act of Parliament to authorise it in 1911, and awarded a contract to Sir Robert McAlpine & Sons on 16 June 1913.

[27] The river continues to flow south-westwards around the hamlet of Trefechan, and as it approaches Cefn-coed-y-cymmer, there is a tramway and leat on the east bank.

Its main function was to provide the ironworks with a good supply of water, but it also fed the ornamental lake at Cyfarthfa Castle through a syphon.

The pollution turned the water white for at least 3 miles (5 km) downstream of the works[29] and killed approximately 23,000 fish, including wild brown trout, bullheads, stickleback and pike, and a large number of invertebrates.

[33] Two modern bridges in central Merthyr Tydfil carry the northbound carriageway of the A4054 across the river to its west bank and back again, with a weir between them.

At Troed-y-rhiw the B4285 Bridge Street crosses, and the Afon Tâf High School if located on the east bank.

It carries a date plaque stating it was built in 1811, but that probably refers to an earlier wooden bridge, and the main arch was rebuilt in 1993.

[35] The river turns to the east to flow around the southern edge of Treharris, and then in a large loop around higher ground on which is Goitre-Coed.

[36] The second was part of the Merthyr Tramroad, and was built in 1815 after the original wooden bridge collapsed into the river when a train crossed it.

The Afon Cynon joins on the right bank, and close to the junction is spanned by a skewed railway bridge with two arches of unequal sizes.

[40] Just to the south is White or Berw Bridge, designed by Pontypridd Urban District Council's surveyor, P R A Willoughby.

It was built in 1907, aided by L G Mouchel & Partners, who pioneered the use of reinforced concrete, and the 115-foot (35 m) span over the river was the longest arch in Britain using this material when it was constructed.

It was built in 1756 by an Independent minister and self-taught civil engineer called William Edwards, and was his third attempt, after his two earlier bridges collapsed.

Isambard Kingdom Brunel's solution was to divert the Taff to the west, creating a larger and safer site for the station.

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.