The river rises at Llyn Cyfynwy near Graianrhyd village in the community of Llanarmon-yn-Ial, Denbighshire, about three miles from the source of the River Alyn.

[1] It then flows northwards and eastwards, forming the boundary between the old parishes of Nercwys and Treuddyn.

At Nant-y-Mynydd it is joined by several small springs from Mynydd Ddu, and finally itself joins the River Alyn at Pontblyddyn, Flintshire.

Its name is derived from its rapid flow after times of heavy rain; Thomas Pennant, in his Tours in Wales, described it as "the Terrig, or the violent, [...] often of a tremendous swell and fury".

This article related to a river in Wales is a stub.