The source of the Mynach is at the meeting of two smaller rivers, the Nant Rhuddnant and the Merin. Both of these streams have their sources in lakes high on the Cambrian Mountains; the Merin flows from Llynoedd Ieuan, and the Nant Rhuddnant from Llyn Rhuddnant.
The Mynach is best known for the famous Mynach Falls in the village of Devil's Bridge (Pontarfynach), where the river plunges 300 feet into the gorge of the River Rheidol. The same village is famous for the three bridges built one on top of the other; the lowest of the three said to be built by the devil (hence the name Devil's Bridge), but most probably built by the monks of Strata Florida.
It may be the monks who give the river its name; the Welsh Mynach means "Monk".