Llyn Cwm Ystradllyn

Llyn Cwm Ystradllyn is a mountain lake in Gwynedd, north-west Wales, lying in a quiet valley above Garndolbenmaen and Porthmadog under the slopes of Moel Hebog. It covers close to 40 hectares and is ringed by moorland, boulder-strewn slopes and the remains of a former slate-quarrying industry. It suits walkers who want an uncrowded corner of the Snowdonia region.

What is Llyn Cwm Ystradllyn?

The water began as a natural lake. The Coflein record, kept by the Welsh national heritage body, describes a dam about 220 metres long that raised the level at the south-west outflow. A Geograph caption dates the present dam to 1959, with later raising in 1976 and 1981. Maximum capacity is roughly 3 million cubic metres. The reservoir feeds water supplies for the Llŷn and Eifionydd areas and is managed by Dŵr Cymru Welsh Water.

What can you see here?

From the dam you look across a sheet of water squeezed between steep valley sides. There is no cafe or visitor centre, so bring food and drink with you.

The valley also carries industrial history. The line of the Gorseddau Tramway runs through it, laid out in the 1850s to move slate down from quarries higher up. A walk from the dam follows this route to Gorseddau quarry, a hollow of cut faces and waste tips. Slate quarries in this part of Gwynedd fall within the UNESCO-listed Slate Landscape of Northwest Wales.

Much older traces survive too. Surveyors have recorded a probable disturbed ring cairn with a central cist, around 2.5 to 3 metres across, in Cwm Ystradllyn, alongside prehistoric hut circles. More cairns are known near the lake shore and on Moel Hebog.

How do you get there?

Drive minor roads from Garndolbenmaen, on the A487 north of Porthmadog. A small car park sits beside the dam. Coordinates are 52.9779 N, 4.1437 W. No public transport reaches the dam, so a car or bike is the practical choice.

When should you go?

Late spring to early autumn gives the driest ground. Sturdy boots matter on the spoil heaps and the old tramway bed, and Welsh upland weather turns quickly, so pack a waterproof. Some old paths are faint, according to visitors, so carry a map.

Frequently asked questions

Is Llyn Cwm Ystradllyn a natural lake? It began as one. A dam raised the water level and turned it into a reservoir.

What is the reservoir used for? It supplies water to the Llŷn and Eifionydd areas and is run by Welsh Water.

Is there parking? Yes, a small car park lies beside the dam.

What else is nearby? Gorseddau quarry, the old slate tramway route, prehistoric cairns and the slopes of Moel Hebog.