The headwaters arise on the southern slopes of Bannau Sir Gaer (Carmarthen Fans) and Fan Brycheiniog and are soon joined by the waters of the Twrch Fechan. The river flows south-west and then south and finally south-east for about 14 km / 9 miles to its confluence with the River Tawe at Ystalyfera. Significant tributaries include the Nant Gwys and the Nant Llynfell.