The River Clwyd is a river in north Wales. It has its rises in the Clocaenog forest (grid reference SJ045535) 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 valley and is joined just south of Denbigh 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 River Elwy flows principally eastwards to join the Clwyd passing through few villages. At the confluence of the River Elwy and the Clwyd, the river becomes tidal and enters a rather unattractive narrow estuary before meeting the Irish Sea at Rhyl.