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 a third of a mile (500 m) 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.