It lies in the Snowdonia National Park at the upper end of Cwm Cowlyd on the south-eastern edge of the Carneddau range of mountains, at a height of 330 metres (1,083 ft) above sea level.

[1] The surrounding hills drop steeply to the water's edge, from Cefn Cyfarwydd and Creigiau Gleision to the east, and Pen Llithrig y Wrach to the west, and as a consequence have not been forested in the 20th century, as were the slopes of neighbouring Llyn Crafnant.

The supply of water to Llyn Cowlyd is assisted at its south-western end by a leat which runs roughly east–west along the 418m contour to the south and west of the lake, along the Llugwy valley.

Llyn Cowlyd can also be reached by foot from Capel Curig, some 5 kilometres (3 mi) away, from the ridge of Cefn Cyfarwydd, or from above Dolgarrog.

These water supply pipes should not be confused with the large (1.77m or 5 ft 10 in diameter) black pipeline which runs for 3 km (5 miles) from the dam to Dolgarrog, and serves the power station there,[2] built in 1925, to support the aluminum works.

During World War II a shepherd called Wil Roberts was paid a nominal sum of money to check the pipeline for bomb damage or any signs of sabotage.

[1] The first dam at Llyn Cowlyd was constructed by the Conway and Colwyn Bay Joint Water Supply Board and completed in 1897.

[3] The present dam at the north-eastern end of Llyn Cowlyd is 14m high (45 feet) consisting of a rock and earth embankment with a concrete core.

Remains today suggest that this was poorly constructed, but its line is easily followed, and is clearly evident from, for instance, the summit of Creigiau Gleision.

According to the Mabinogion, the most ancient of Celtic literature written in the 14th century in the Red Book of Hergest, but orally dating back much further, the area was inhabited by more wildlife than seems to be there today.

One of the tasks is to find the lost Mabon, son of Modron, and a number of mythical beasts are consulted, one of whom is the Owl of Cwm Cowlyd.

The owl narrates the history of its Cwm, and if any of it is to be believed, it confirms that the valley was once wooded (as was most of Snowdonia), but that the first clearing took place much earlier than mediaeval times, which is more unusual.

"Owl of Cwm Cawlwyd, here is an embassy from Arthur; knowest thou aught of Mabon the son of Modron, who was taken after three nights from his mother?"

The Eagle said, "I have been here for a great space of time, and when I first came hither there was a rock here, from the top of which I pecked at the stars every evening; and now it is not so much as a span high.

After that I went with my whole kindred to attack him, and to try to destroy him, but he sent messengers, and made peace with me; and came and besought me to take fifty fish spears out of his back.

Another myth is that of the water bull who appears from the depths with "fiery horns and hoofs with flames issuing out of its nostrils".

The poet R. S. Thomas makes reference to the Mabinogion's Owl of Llyn Cowlyd in his poem "The Ancients of the World".