The Ystrad River flowed from a source in the Eagle Mountains of northern Prydain, southward between the Hill Cantrevs to the east and the Forest of Idris to the west, through the Ystrad Valley (home of the Valley Cantrevs), and finally spilled into the Great Avren River.
The Ystrad was shallow during the dry season between summer and autumn, and could be easily crossed at most locations, but it was deeper and more turbulent after the rains during autumn and spring.
The Ystrad and its adjacent Valley are a major setting in the first, fourth and fifth books of the Chronicles of Prydain.
The events transpiring from the imprisonment of Gwydion and Taran at Spiral Castle in Chapter 5, through Chapter 13 when the Companions enter Medwyn's Valley, take place either within the Ystrad Valley or on a ridge overlooking it. Later, in Chapter 17, the group sees the forces of the Horned King marching towards Caer Dathyl, and re-enters the Valley for the climactic events of Chapter 18.
Once again, all events taking place in Cantrev Cadiffor, from the Companions' arrival at Caer Cadarn in Chapter 3 through the death of Rhun and the separation of the bands in Chapter 6, occur within the Valley.