The Nant Llech Isaf and the Nant Llech Pellaf combine at Blaen-Llech to form the Nant Llech (English: 'rock-slab stream').  In its short dash to join the Afon Tawe near Ynyswen it leaps over the Farewell Rock to provide the highest waterfall in Fforest Fawr Geopark and indeed one of the highest in South Wales.

The Henrhyd Falls occur where this band of resistant sandstone is faulted against easily eroded mudstones.  The site is now owned and managed by the National Trust. 

A path follows the river for the rest of its course down a delightful wooded gorge to Llech Bridge and its confluence with the Afon Tawe.