The artificial lake, which was constructed between 1973 and 1976, was created by building an embankment dam across the Afon Brenig valley.

The reservoir's purpose is to protect the water supply for North West England and north-east Wales, particularly Liverpool and its surrounding area.

An earthen bank with stone, which is up to 45 m (148 ft) at its highest point, dammed the Afon Brenig near the village of Pentre-Llyn-Cymmer, Cerrigydrudion.

Llyn Brenig is a popular destination for outward bound activities such as hiking and mountain biking, fishing and watersports.

[5] The same pair of ospreys returned to Llyn Brenig in 2022 and hatched chicks at a replacement nest in the same location.