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.