It has its rises in the Clocaenog forest five miles north west of Corwen. It flows due south until at Melin-y-Wig it veers northeastwards, tracking the A494 to Ruthin. Here it leaves the relatively narrow valley and enters a broad agricultural vale, the Vale of Clwyd (Welsh: Dyffryn Clwyd). Just south of Denbigh it is joined by the River Clywedog. This substantial tributary also has its source in the Clocaenog forest but drains out to the east and north of the forest and passes through Cyffylliog, Bontuchel and Rhewl before its confluence with the main river.
Then the Clwyd meanders northwards through the fertile Clwyd valley to St Asaph. Two miles north of St Asaph, the river is joined by a tributary as large as the main river, the River Elwy. In normal flows at low tide, the waters of these two rivers can be seen flowing side by side down the river with little mixing.
The River Elwy has its source a long way to the west on the flanks of Moel Seisiog south-east of Llanrwst. It also has a tributary the River Aled with its source in the upland lake Llyn Aled. The Elwy flows principally eastwards to join the Clwyd passing through few villages. At the confluence of the Elwy and the Clwyd, the river becomes tidal and enters a narrow estuary with lots of wildlife before meeting the Irish Sea at Rhyl.
In 1277 King Edward I sited a castle next to the Clwyd at Rhuddlan. In order for his supply barges to reach the castle, he employed 968 men to work on what was called 'the great ditch'. He straightened the three-mile stretch of river that ran from the castle to the sea. This was an enormous feat of engineering, compared even to today's modern standards, and it is still possible to make out the original loops and turns of the Clwyd using satellite imagery.