This dam is the biggest in Wales with a road running along the top.
Wikipedia describes Llyn Celyn:
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'Llyn Celyn is a large reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in 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 71.2 billion litres of water.
It was originally to be called Llyn Tryweryn Mawr, but in September 1964 Liverpool Corporation agreed to the name change following a letter by the Tryweryn Defence Committee.
Construction and opposition
Construction of the reservoir involved flooding the village of Capel Celyn and adjacent farmland, a deeply controversial move. Much of the anger was occasioned because the village was a strong-hold 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 wisdom 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.”
The reservoir was constructed in order to support the water abstractions from the River Dee as part of the River Dee regulation system, including the abstraction by North West Water at Huntington water treatment works, Chester, from where water is supplied to Liverpool and the Wirral. The reservoir is contained behind a rock gravity dam and, at its upper end, it is squeezed between Arenig Fawr and Arenig Fach, two of the mountains of south Snowdonia.
Water is released from the reservoir into the River Tryweryn which then flows into the River Dee. Most releases pass 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 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 throttled back to a minimal maintenance flow.
Diversions/closures of transport links
The building of the reservoir also contributed to the final closure of the GWR branch line from Bala to Blaenau Ffestiniog.[4] Passenger trains had ceased 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.'