Солтүстік Уэльстегі Ллин Коулид көлі

Llyn Cowlyd is the deepest lake in northern Wales. 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 above sea level. The lake is long and narrow, measuring nearly long and about 500 m (third of a mile) wide, and covers an area of It has a mean depth of and at its deepest has given soundings of , this being some greater than its natural depth, the water surface having been raised twice by the building of dams. 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. Indeed, there is not a tree to be seen, and the general aspect is one of bleakness. Dependent on the weather conditions, the waters often appear dark. 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. It is also fed by water from Llyn Eigiau. Llyn Cowlyd can be reached by road from Trefriw, some to the east, although the metalled road stops at a gate, the best part of a mile from the lake itself, beyond which private vehicles are not permitted. Llyn Cowlyd can also be reached by foot from Capel Curig, some away, from the ridge of Cefn Cyfarwydd, or from above Dolgarrog. A good path runs along the north western shore of the lake. The stream which flows from Llyn Cowlyd is called Afon Ddu. This flows into the river Conwy, passing Pont Dolgarrog on the B5106 road, just south of the village of Dolgarrog. The gorge cut by the river at this point is popular for gorge walking.

Some sources state that the name Cowlyd comes from "Cawlwyd" or "Cawllwyd". Algernon Herbert's book Britannia after the Romans refers to Caw, Lord or Chieftain of Cwm Cawllwyd. "Cwm Cawllwyd" could therefore translate as The Cwm of Grey Caw. Further research suggests that the name might derive originally from Caw ap Geraint Llyngesog ab Erbin ap Custennin Gorneu ap Cynfor ap Tudwal, the famous warrior who features in Culhwch and Olwen. Caw's son Celyn ap Caw had a watchtower, Tŵr Celyn, near the Copper mountain, on Anglesey. The Ordnance Survey map of 1841 records the name as "Cwlyd", as does the National Gazetteer of 1868. 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. His only find was a crashed Anson aircraft in February 1944. The water mains from Cowlyd are interconnected by means of cross overs at various points with those which were laid by the Llandudno Improvement Commissioners (later duplicated by the Llandudno Urban District Council) from their source, Llyn Dulyn. This was to provide continuity of supply to each reticulation in case of emergency. In 1965, local water undertakings were grouped together under the control of the Conway Valley Water Board, whose Engineer and Manager, Joseph M. Campbell, was the last Engineer to Conway and Colwyn Bay Joint Water Supply Board. Since the 1990s, the Llandudno system has been supplied from Cowlyd when Dulyn transferred for generating purposes. Llyn Cowlyd is one of two reservoirs which supply hydro electricity to the aluminium works at Dolgarrog. The other is Llyn Eigiau, which was physically connected to Llyn Cowlyd by a tunnel in 1919, and this is the water which flows into the lake by the dam. Water also flows from Llyn Eigiau more directly to Dolgarrog via Afon Porth llwyd and Coedty reservoir. Llyn Eigiau lies a mile to the north of Llyn Cowlyd, and its water rights were at the time owned by the Aluminium Corporation Limited. The history of electricity production at Dolgarrog is tied up with that of Henry Joseph Jack (1869 1936), who was chairman of the North Wales Power and Traction Co. Ltd, and who had a dream of using this new power source to power the narrow gauge railways in North Wales. The NWPTC was the company behind the Portmadoc, Beddgelert and South Snowdon Railway (to later become the Welsh Highland Railway) and he became Chairman of the Festiniog Railway during July 1921, putting him in control of all the passenger carrying narrow gauge railways of that part of North Wales. In addition to this, in 1922 he became a director of the Snowdon Mountain Railway which was also part of the Dolgarrog empire. 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. A concrete and stone apron protects the dam from wave action, which comes from the direction of the prevailing winds. The dam, which was built some 274m (300 yards) downstream of the lower, earlier one, was completed in 1921, and was officially opened on 20 September 1922. During construction of Cowlyd dam, electricity for the winches was provided by the North Wales Power and Traction Co. Ltd's power station at Cwm Dyli, near Snowdon. The remains of the pole route along the western shore of the lake can be seen today. Stone for the dam came from an adjacent quarry, some 1/4 mile to the north of the dam site. A short incline was constructed for transfer of stone, and it ran directly from the quarry down to the dam. 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. A later vehicle route also winds from the dam to the quarry. Remains of the winding drum can still be seen. Construction workers lived in wooden barracks, a number of which afterwards found their way back to the Trefriw and Dolgarrog area to be converted into homes, and are still lived in.

Construction of the dam was assisted by the construction of a narrow gauge 2 ft (610 mm) gauge railway line, known as the Llyn Cowlyd Tramway, which was used to convey men and materials to the reservoir for construction and subsequent maintenance purposes. The line ran from near Coedty reservoir, above Dolgarrog, up as far as the dam. This tramway was essentially a branch line of the Eigiau Reservoir Tramway, laid to aid construction of the dam at nearby Llyn Eigiau.

During a storm on 31 December 1924, there was a partial failure of the dam when the downstream side was washed out. The damage was kept fairly secret, and rebuilding it provided more useful work for the tramway. This partial dam failure of the Cowlyd Dam should not be confused with that of the Dolgarrog Disaster of 1925, when the failure of the Eigiau Dam released water which went on to overtop the Coedty Dam, above Dolgarrog. This second dam also failed, releasing the huge volume of water that flooded Dolgarrog, killing 16 people.

Fly fishing (by permit) is permitted in the lake, which contains brown trout and Arctic char, originally taken from Llyn Peris, in Llanberis, when that lake was drained as part of the construction of Dinorwig Power Station.

The poet R. S. Thomas makes reference to the Mabinogions Owl of Llyn Cowlyd in his poem "The Ancients of the World". Gwilym Cowlyd (1828–1904), whose real name was William John Roberts, was a native of Trefriw, and one of the more colourful figures in Welsh culture and the Welsh Eisteddfod. His bardic name was taken from neighbouring Llyn Cowlyd.