Llyn Cwellyn supplies drinking water to parts of Gwynedd and Anglesey. It has an area of 215 acres (0.87 km2), and is over 120 ft (37 m) deep. Although it is now dammed at the northern end, below the Castell Cidwm ridge, near the village of Betws Garmon this has not substantially increased the size of the natural glacial moraine lake that has existed since the last ice age. The lake is very deep and is one of the few lakes in Wales to support a natural population of Arctic char. The lake also is home to brown trout and there are otters that are regularly seen at the Castell Cidwm end.