Llyn Eigiau is a lake on the edge of the Carneddau range of mountains in Snowdonia, Conwy, Wales.

The original contractor pulled out of the construction, alleging corner-cutting, and indeed on 2 November 1925, following 26 inches (660 mm) of rain in just five days, the dam broke.

[1] A study of the dam today shows that the foundations were insufficient, and large lumps of unmixed cement can also be seen.

The outflow from Llyn Eigiau is called Afon Porth-llwyd, and this flows via Coedty reservoir before passing under Pont Newydd in Dolgarrog.

Although private vehicles are not permitted access to the lake itself, there is a carpark within walking distance, approx.