[7] Tal-y-llyn Lake is situated to the north of Machynlleth,[8] at the foot of Cadair Idris, in the Snowdonia mountain range of Gwynedd, Wales.

The River Dysynni flows from the lake, through the village of Abergynolwyn, and discharges into the sea north of Tywyn.

[9][10] There is a route leading to the summit of Cadair Idris from near the lake and the narrow gauge Talyllyn Railway has its eastern terminus at nearby Nant Gwernol near the village of Abergynolwyn, within the parish of Tal-y-llyn, the hamlet at the southern end of the lake.

It is now understood that what appeared to be bedrock is in fact massive blocks of debris left by a large landslide.

1976 Newbery Medal winner Susan Cooper used the lake as a setting for the book The Grey King.