Llyn Celyn - Operation of Reservoir

Operation of Reservoir

The reservoir was constructed in order to help maintain the flow in the River Dee, so that drinking water could be abstracted further downstream as part of the regulation Scheme. These abstractions include one at Huntington water treatment works Chester operated by North West Water, which supplies water to Liverpool and the Wirral.

The reservoir is contained behind a rock gravity dam and, at its upper end, it runs between Arenig Fawr and Arenig Fach, two of the mountains of south Snowdonia.

Water is released from the reservoir into the River Tryweryn which flows into the River Dee. Most of the water passes through a small hydro-electricity plant to supply green electricity to the National Grid. The released water first flows into a stilling basin and then down the narrow and rocky valley of the River Tryweryn. This section of the river provides facilities for international level white-water canoeing, and rafting at the Canolfan Tryweryn National White-water Centre. Some water in the reservoir is held in reserve to make special release down the river for specific white-water events. Because the reservoir's principal purpose is to support low river flows in the main River Dee, the best conditions for white-water occur during long dry spells in summer when maximum releases are made. Usually the dam will release between 9 and 11 m³/s although releases as low as 2 m³/s and as high as 16 m³/s have been known. During wet weather the releases are usually throttled back to a minimal maintenance flow unless a planned release for recreational activities has been agreed.