Llyn Cwellyn (Llyn Quellyn in some antiquated texts) is a reservoir in North Wales which supplies drinking water to parts of Gwynedd and Anglesey. It lies between the Snowdon Massif and Mynydd Mawr in the northern part of Snowdonia National Park. It has an area of 215 acres (0.87 km²), and is over 120 ft (37 m) deep. At the southern end is the small village of Rhyd Ddu

Although it is now dammed at the northern end, this has not substantially increased the size of the natural glacial moraine lake that has existed since the last ice age. The lake is very deep and is one of the few lakes in Wales to support a natural population of Arctic char.

The Lake is passed the whole of its length by the A4085, which borders it so closely, at places the tarmac is being undermined. Running the length of the lake, although further up the mountainside is the Welsh Highland Railway, which offers splendid views of the lake looking North.

The lake water has been implicated in an outbreak of Crypyosporidiosis in December 2005 in the catchment that it supplies but this has not yet been confirmed.

Reference

• The Lakes of Eryri by Geraint Roberts. Gwasg Carreg Gwalch, Llanrwst, 1995.

Lakes of Wales | Reservoirs in Wales | Geography of Gwynedd

Llyn Cwellyn