Llyn Cwellyn is a reservoir

Reservoir

A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 in North Wales

North Wales

North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...

 which supplies drinking water

Drinking water

Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 to parts of Gwynedd

Gwynedd

Gwynedd is a county in north-west Wales, named after the old Kingdom of Gwynedd. Although the second biggest in terms of geographical area, it is also one of the most sparsely populated...

 and Anglesey

Anglesey

Anglesey , also known by its Welsh name Ynys Môn , is an island and, as Isle of Anglesey, a county off the north west coast of Wales...

. It lies on the Afon Gwyrfai in Nant y Betws between the Snowdon Massif

Snowdon Massif

The Snowdon Massif is one of the three mountain groups in Snowdonia, north Wales, to include mountains over 3,000 feet high. It occupies the area between Beddgelert, Pen-y-Pass and Llanberis...

 and Mynydd Mawr

Mynydd Mawr

Mynydd Mawr is a mountain in Snowdonia, North Wales, situated approximately west of Snowdon itself, overlooking Llyn Cwellyn and being the northern outlier of the Moel Hebog group. A popular ascent starts from Rhyd Ddu. On its western flank are the remains of ancient settlements and field systems...

 in the northern part of Snowdonia National Park. It has an area of 215 acre (0.8700749 km²), and is over 120 ft (36.6 m) deep. At the southern end is the small village of Rhyd Ddu

Rhyd Ddu

Rhyd Ddu is small village in Snowdonia, North Wales.Rhyd Ddu is well known as a popular starting point for walks up Snowdon , Moel Hebog, Yr Aran and the Nantlle Ridge....

.

Although it is now dammed at the northern end, below the Castell Cidwm ridge, near the village of Betws Garmon

Betws Garmon

Betws Garmon is a community and small hamlet outside Waunfawr and near Beddgelert in Gwynedd, Wales. It has a population of 216.The summit of Snowdon lies within the community boundaries. Bryn Gloch has the newly reopened narrow gauge Welsh Highland Railway passing alongside it.Over the road from...

 this has not substantially increased the size of the natural glacial moraine

Moraine

A moraine is any glacially formed accumulation of unconsolidated glacial debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past glacial maximum. This debris may have been plucked off a valley floor as a glacier advanced or it may have...

 lake that has existed since the last ice age.