Construction began in 1973 and was completed in 1976. An earthen bank with stone, which is up to at its highest point, dammed
Afon Brenig near the village of
Pentre-Llyn-Cymmer,
Cerrigydrudion. The northern half of the reservoir is in
community of
Nantglyn. Its perimeter is about . Brenig, which has a capacity of 60 million m³, was first filled in 1979. Water supply of the catchment into the lake is significantly "over-reservoired", this means the catchment area of Llyn Brenig does not have capacity to refill the reservoir within one annual hydrological cycle. It can take several years for the reservoir to refill again after its water level have been drawn down. Water is only drawn from Llyn Brenig during drought conditions when
Llyn Celyn and
Bala Lake are no longer predicted to be capable of maintaining the flow in the River Dee. The lake holds a commercial trout farm and commercial logging is conducted around parts of the lake. ==Leisure ==