Its source is north of the village of Crymych and its length is about 11 miles (18 km) to its estuary at Newport, Pembrokeshire.

The source of the river is a spring on the southwest slope of Frenni Fawr, near Crymych.

It flows southwest for a short distance, then to the north and then generally west, skirting the northern slopes of the Preseli Hills.

[1] The river is bridged by several lanes and the B4329 Eglwyswrw to Crosswell road before passing beneath the A487 at Felindre Farchog.

The river discharges into the sea at Newport's old port area of Parrog where it is possible to ford the river on foot, with caution, at low tide.