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 on the Afon Gwyrfal in Nant y Betws 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, near the village of Betws Garmon 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 water has been confirmed as the source of an outbreak of Crypyosporidiosis in November-December 2005. Contamination appears to have occurred from septic tanks or cess pits in the catchment.
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 from the south. Road and railway both run from Caernarfon to Beddgelert and then through the Aberglaslyn Pass to Porthmadog. They come together at Rhyd Ddu railway station at the start of the Rhyd Ddu path to the summit of Snowdon.
Rail and road run side by side for over half a mile to the summit of the mountain pass at Pitt's Head where the road passes over the railway before descending steeply through Nant Colwyn to Beddgelert. Pitt's Head is also the starting point of the 'Beddgelert' branch of the Rhyd Ddu path to Snowdon. Midway along the eastern shore of the lake is the starting point of the easier but less sensational Snowdon Ranger path to the summit of Snowdon.
Since the Middle Ages the overland route from Caernarfon to Beddgelert via Llyn Cwellyn and then via the River Glaslyn and the Traeth Mawr to Cardigan Bay was often the preferred alternative to the long voyage round the Lleyn Peninsula.