For much of its short length it marks the border between England and Wales. The river rises near Craswall on Cefn Hill. It flows southwards, gaining the waters of its tributaries Escley Brook and Olchon Brook near Clodock and the waters of Afon Honddu, from the Black Mountains, near Pandy. The river then flows briefly eastwards before again turning southwards. At Monmouth the river joins into the River Wye with the River Trothy. Its total length is around 26 miles (42 km).
At one time the river was noted for its substantial brown trout population, with the length from Pontrilas to Skenfirth producing record catches. Numbers fell substantially during the 20th century and especially after the 1960s.
Monnow Bridge is the only remaining mediaeval fortified river bridge in Great Britain with its gate tower still standing on the bridge.
It is located in the town of Monmouth, which stands at the confluence of the Wye and the Monnowrivers.
The gatehouse on Monnow Bridge called Monnow Gate, which gives it is remarkable and noteworthy appearance, was added to the bridge in the 14th century.
It flows southwards, gaining the waters of its tributaries Escley Brook and Olchon Brook near Clodock and the waters of Afon Honddu, from the Black Mountains, near Pandy.
At one time the river was noted for its substantial brown trout population, with the length from Pontrilas to Skenfrith producing record catches.