Llyn Eigiau is a lake on the edge of the Carneddau range of mountains in Snowdonia, North Wales.
The name Eigiau is thought to refer to the shoals of fish which once lived here. Early maps refer to it as Llynyga. It is thought that a small number of Arctic Char exist in the lake (as they do in neighbouring Llyn Cowlyd) after they were transferred here from Llyn Peris, and certainly it is one of the few lakes in Wales to have its own natural brown trout.
In 1911 a dam ¾ mile long and 35 foot high was built across part of its eastern side to supply water for the power station at Dolgarrog, which in turn provided power for the adjacent aluminium works. The original contractor pulled out of the construction, alleging corner cutting, and indeed on 2nd November 1925, following 26 inches of rain in just five days, the dam broke. The water flowed down to Coedty reservoir, also causing that to burst, and millions of gallons of water flowed down into the village of Dolgarrog, causing the loss of 17 lives. A new power station was built at Dolgarrog in 1925. Black & white silent film of the incident can be seen here
A study of the dam today shows that indeed the foundations were quite insufficient, and large lumps of unmixed cement can also be seen.
Today the lake covers an area of about 120 acres, and has a depth of about 32 foot. After the construction of the dam its area would have been twice this.
Water is fed into Llyn Eigiau by a tunnel from the stream below Llyn Dulyn, and another larger tunnel takes water from Llyn Eigiau to Llyn Cowlyd.
The main feeder of Llyn Eigiau is Afon Eigiau, a small river which flows down Cwm Eigiau.
The outflow from Llyn Eigiau is called Afon Porth-llwyd, and this flows via Coedty reservoir before passing under Pont Newydd in Dolgarrog. This then flows into the River Conwy.
Although it is not possible to reach the lake by private vehicle, there is a car park some half a mile from the dam, reached by narrow lane from Tal-y-bont, in the Conwy valley.
The construction of Eigiau dam was facilitated by the construction of a tramway from Dolgarrog. This largely followed the line of the former Cedryn Tramway (1861-1888), a narrow gauge 4 mile long horse-powered tramway which served the Cedryn slate quarry, a little to the south-west of Llyn Eigiau. Between the top of the Dolgarrog incline (just below Coedty reservoir) and Llyn Eigiau itself the route was converted to standard gauge (from about 1907), although it was subsequently relaid in narrow gauge (from about 1916) when the Cowlyd Tramway was begun. In its years as a standard gaue railway, steam locomotives used the line, and were hauled up from Dolgarrog using the incline now occupied by the water pipelines.
Today, whilst the line of the tramway can easily be followed, and makes a pleasant walk from Eigiau down to Coedty reservoir, there is no evidence of rails or sleepers of any kind. The loading gauge is quite clearly that of a standard gauge railway, and in places there is evidence of minor embankments, cuttings, and a bridge. Towards Llyn Eigiau the Tramway runs along a level path, though the gradients are greater through the woods immediately above Coedty.
The Cowlyd Tramway branched off from the Eigiau tramway at the top of the Dolgarrog incline. Eigiau tramway followed a route to the south of Afon Porth-llwyd, and largely parallel to it.