|Basin countries||United Kingdom|
|Max. length||2.5 mi (4.0 km)|
|Max. depth||140 ft (43 m)|
Llyn Celyn is a large reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in Gwynedd, North Wales. It measures roughly 2½ miles long by a mile wide, and has a maximum depth of 140 ft (43 m). It has the capacity to hold 71,200 megalitres of water.
Construction and opposition 
Construction of the reservoir involved flooding the village of Capel Celyn and adjacent farmland, a deeply controversial move. Much of the opposition was brought about because the village was a stronghold of Welsh culture and the Welsh language, whilst the reservoir was being built to supply Liverpool and parts of the Wirral with water, rather than Wales. The legislation enabling the development was also passed despite the opposition of 35 out of 36 Welsh Members of Parliament, with the 36th not voting. This led to an increase in support for the Welsh nationalist party, Plaid Cymru, in the late 1950s and gave fresh impetus to Welsh devolution.
Although many doubted the need of having an official opening, this took place on 21 October 1965. Representatives came from Liverpool council, and invitations were sent to all those with family links to the valley. In view of the anticipated protest, there was a strong police presence. The ceremony in fact lasted less than 3 minutes, for protesters had cut the microphone wires, and the chants of the hundreds of protesters made the speeches inaudible.
In October 2005, Liverpool City Council passed a public apology for the incident.
The full statement reads -
The Council acknowledges its debt to the many thousands of Welsh people who have made their homes in the City. They have, in so many ways, enriched the life of the City.
We know that Liverpool, especially in the fields of medicine and education, has been of real service to the people of Wales.
We realise the hurt of forty years ago when the Tryweryn Valley was transformed into a reservoir to help meet the water needs of Liverpool.
For any insensitivity by our predecessor Council at that time, we apologise and hope that the historic and sound relationship between Liverpool and Wales can be completely restored.
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.
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.
The building of the reservoir also contributed to the final closure of the GWR branch line from Bala to Blaenau Ffestiniog. Passenger trains had ceased running in 1960, and the last freight train ran in 1961. The line was subsequently flooded by the lake, and the base of the dam also crosses it. Liverpool Council had in fact planned a railway diversion, but this was never built as the British Transport Commission had decided to close the line. As a result of this, Liverpool Council decided to contribute towards the cost of the new main road (the A4212, which was built across the pass from Bala to Trawsfynydd around the north side of the lake), and also towards the cost of a line linking the two stations in Blaenau Ffestiniog.
See also 
- National Library of Wales page, "Celyn lake held a capacity of 71,200 mega litres of water, the biggest dam in Wales"
- Capel Celyn, Ten Years of Destruction: 1955 - 1965, by Einion Thomas, published by Cyhoeddiadau Barddas & Gwynedd Council, 2007, ISBN 978-1-900437-92-9
- Flooding Apology Where I Live:Liverpool, BBC website
- http://www.penmorfa.com/Conwy/six.htm penmorfa.com