Llyn Brenig is a reservoir located in North Wales, in the heart of the Denbigh moors, on the border between the counties of Conwy and Denbighshire. It is used to manage the flow in the River Dee, Wales as part of the River Dee regulation system which is designed to protect the water supply for North West England and North East Wales, particularly Liverpool and the surrounding area.
Construction began in 1973 and was completed in 1976. It has a capacity of 60 million m≥ and was first filled in 1979. Whilst it is not the largest lake in Wales in terms of volume (that title belongs to Bala Lake/ Llyn Tegid), it is the largest in terms of area covered, around 920 acres. It has a perimeter of some 14 miles.
The catchment of the reservoir is very significantly over-reservoired. This means that the reservoir cannot fill from its own catchment within one annual hydrological cycle. When the reservoir level is drawn down it can take several years for it to completely re-fill again. Llyn Brenig is therefore only used during drought conditions when the capacity of Llyn Celyn and Llyn Tegid are no longer predicted to be capable of maintaining the flow in the River Dee.
During construction of the lake, a number of Bronze Age artifacts were found, as was a camp used by Mesolithic hunter/gatherers. This has been dated by radiocarbon analysis of the charcoal from their fires to around 5700 BC. There are a number of archaeological trails around Llyn Brenig; the relics visible include a ring cairn (a Bronze Age burial mound), and several barrows.
A visitor centre to the south of the lake displays a lot of archaeological information about the area.
A number of watersports take place on the lake, including sailing, windsurfing, rafting and canoeing. The Trust enforces a 10 mph limit on the lake, restricting the use of jet-skis and power boats.
Access is via the B4501, turning north off the A5 at Cerrig-y-drudion.