Grwyne Fawr
The Grwyne Fawr is a small river in south-east Wales.
It rises at the slopes of Rhos Dirion in the Black Mountains. In its upper course is the Grwyne Fawr Reservoir, as the river flows south-east. It continues to flow south-east, forming part of the border between Powys and Monmouthshire. It then turns west at Forest Coal Pit and joins the Grwyne Fechan near Llanbedr Ystrad Yw. From there the river continues south past Llangenny and joins the River Usk at Glangrwyney.
The origin of the name "Grwyne" is uncertain, but it is likely from 'gweryn' or 'gwerynau', meaning wet place. "Fawr" means big and is to distinguish from the smaller Grwyne Fechan ("fechan" = small).[1]