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.