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.

After passing through Bont-newydd, the river turns northwards again and flows through St. Asaph (Llanelwy or "the church enclosure on the Elwy" in Welsh).

A number of caves along the lower valley of the Elwy are of great archaeological interest and are considered one of the most important groups of Palaeolithic and later caves and rock shelters in Britain.

[1] The Elwy is also well known for its sea trout (sewin) fishing, and also has a run of Atlantic Salmon.

In 2017 a wild trout weighing 5½ lbs was caught and released on a river Elwy beat owned by the angling association.