(often written "Cuch") is a small river tributary to the River Teifi
in south-west Wales
. Its "official" source (although not its highest headwater) is at Blaencych . It flows north-westwards through a deep, wooded, secluded valley, and joins the River Teifi at Abercych
. Its total length is 13 km. It receives numerous small tributaries: the Sylgen, Barddi, Mamog, Dwrog and Lŵyd on the east side, and the Pedran, Cneifa and Dulas on the west side. It formed the ancient boundary between the commotes
Is Cuch and Emlyn Uwch Cuch, and it today forms part of the boundary between Pembrokeshire
The valley (Glyn Cych) is well-known in Welsh literature as the place where, in the Mabinogion, Pwyll Pendefig Dyfed has his fateful meeting with Arawn, Lord of the Underworld. Boundary streams were often thought of as portals to the underworld.