The River Taff () is a river in Wales. It rises as two rivers in the Brecon Beacons; the Taf Fechan (little Taff) and the Taf Fawr (great Taff) before becoming one just north of Merthyr Tydfil. Its confluence with the River Severn estuary is in Cardiff. The river supports several species of migratory fish, including salmon, sewin (sea trout), and eel. Course From its confluence at Cefn-coed-y-cymmer, the river flows south, passing several towns. It picks up a few tributaries, such as the River Cynon, River Rhondda, Bargoed Taf and Nant Clydach. It flows through Pontypridd and through to Taff's Well, the site of Wales' only thermal spring. It flows underneath the M4 Motorway, before turning southeastward and flowing past the Cardiff suburbs of Radyr, Whitchurch, Llandaff, Pontcanna, the city centre and Grangetown, before emptying into Cardiff Bay, near to the mouth of the River Ely. Taf Fawr 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, 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. 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. It allowed the corporation to build three reservoirs. The catchment of the upper Taf Fawr was but it was split into an upper area of and a lower one of . Any works had to provide compensation water to maintain the flow in the Taf Fawr, and working on the upper area first meant that the Corporation only had to supply per day of compensation flow until work started on the lower area. Cantref Reservoir, the lower of the two in the upper area, was constructed first. Work began on 4 May 1886 and was completed on 14 September 1892. Construction of Beacons Reservoir, the highest in the chain, began shortly afterwards, in April 1893, in advance of the Corporation obtaining a second Act of Parliament to increase the capacity of the reservoir and to move the dam further upstream, where the foundations would be better, in the light of geological investigation. Impounding of the water began on