It lies on the Afon Gwyrfai in Nant y Betws between Moel Eilio and Mynydd Mawr in the northern part of Snowdonia National Park.

Although it is now dammed at the northern end - below the Castell Cidwm ridge, 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 has shared ownership between Lawrence Jones (businessman), owner of the Castell Cidwm estate at the northern end, and Welsh water, who own the right-hand side which supplies the weir.

The A4085 runs past the lake; however, there are no lay-bys for picnics and the east side is now considered a Nature Reserve, and fishing is not permitted in the tree-lined area beside the road all the way down to Castell Cidwm.

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 Llŷn Peninsula.