Llyn Elsi is a lake located above the village of Betws-y-coed in the Snowdonia National Park in North Wales. It is a reservoir providing water for the village. There is a path round the lake, and affords good views to the north-west towards the mountain ranges of the Carneddau and the Glyderau. Snowdon itself is not visible behind Moel Siabod. The lake is more than 700 ft above sea level, and has a small island. In 1946 black-headed gulls established a colony at the lake, causing concern over pollution to the water supply.