Rivers

A river is a natural watercourse, usually freshwater, flowing down from an upland source towards an ocean, a lake, a sea, or another river. In the land of Prydain there are six named rivers, all of which flow from the mountainous uplands in the north of the land, down through the lowlands and valleys of the south.

(The river Alaw on the Isle of Mona also conforms to this up-down layout, though it is a much smaller river on insular terrain.)