The source of the river is sometimes said to be on the northern flank of
Moel Seisiog, south-east of
Llanrwst, at
Ordnance Survey grid reference SH 853593. However the river only receives the name Elwy at the village of
Llangernyw, where three rivers, Afon Cledwen, Afon Collen and Afon Gallen, meet to form the Elwy. It flows eastwards through
Llanfair Talhaiarn and a few miles downstream from this village it is joined by a tributary, the
River Aled () which has its source in
Llyn Aled. A 200-metre section of the river north of
Llannefydd () was in the news in 2026 after its course had shifted dramatically to the south in the space of a few years. After passing through
Bont-newydd, the river turns northwards again and flows through
St Asaph (, ). It joins the River Clwyd about halfway between St Asaph and
Rhuddlan, and the waters of the two rivers can often be seen flowing side by side for several miles. ==Caves and archaeological remains==