17 September 1897. 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 , almost double that specified in 1884. Impounding of water began on 23 June 1926, and the reservoir was full by November, but formal commissioning did not take place until May 1927. Once built, it supplied of compensation water per day to maintain the flow in the river. Below Llwyn-on, the river is crossed by the A470 Brecon to Merthyr road, the A465 Heads of the Valleys road, and as it passes to the south-west of Cefn-coed-y-cymmer, the Cefn Railway Viaduct. which consists of 15 arches built on a curve. 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. It is grade II* listed as its history and architecture surpass that of most other railway viaducts in Wales. On the west bank are the remains of a leat, constructed in 1766 for the Cyfarthfa Ironworks. It is fed by a stone weir with a cast iron sluice. Parts of the leat are cut into solid rock, and parts have stone walls and buttresses. The river is joined by the Taf Fechan near Cyfarthfa Castle. Taf Fechan The Taf Fechan rises immediately to the south of Pen y Fan and runs southwards through three reservoirs and a former fourth, now abandoned. Whereas the Taf Fawr was the preserve of Cardiff Corporation, the Taf Fechan was used to supply water to Merthyr Tydfil Corporation. Of the Upper Neuadd and Lower Neuadd reservoirs, the lower one was built first, authorised by an Act of Parliament obtained in 1876. Like those on the Taf Fawr, it had an earth embankment with a clay core. The clay had to be imported, and for this purpose a private branch line was built from the Brecon and Merthyr Tydfil Junction Railway near Torpantau railway station. The contract for the work was given to William Jones of Neath, and the project was completed in 1884. Now decommissioned, the water level when the reservoir was full was at above Ordnance Datum, and it held . The Upper Neuadd Reservoir is rather larger, and was authorised by an Act of Parliament obtained in 1895. The dam which is high was constructed of