The rock gravity dam, viewpoint of this photo, er, dams the Afon Tryweryn, trapping up to 74.5 billion litres of water as an artificial lake 4km by 1½ km by 42 m deep. It was constructed between 1960 and 1965 to supply Liverpool and the Wirral (and the part of NE Wales where I grew up) with water, not by pipeline but by regulating the flow of the River Dee – abstraction is done near Chester.
A secondary function is to generate electricity via a small hydropower plant, whilst a third is to serve Canolfan Tryweryn, the National Whitewater Centre: extra water – up to 16 cumecs (16 cubic metres (16,000 litres) per second!) – can be released on demand for kayaking events.

Llyn Celyn also played an important role in Welsh nationalist politics.

Construction of the reservoir required the flooding of not only farmland but the small village of Capel Celyn, an acknowledged stronghold of Welsh culture, in order to supply England with water. Worse, Liverpool City Council obtained authority in 1957 via an Act of Parliament, thereby cutting the relevant Welsh Local Authorities out of the planning consent process and diluting the influence of Welsh Members of Parliament, 35 (of 36) of whom opposed the legislation (the 36th didn't vote).
An unsuccessful eight-year campaign by the local community ended with the flooding of twelve houses and farms in 1965, plus the village post office, school and church, including the cemetery. Of the 67 people living in the Tryweryn Valley, 48 lost their homes.
This demonstration that Wales had negligible powers of self-determination – an English council had got its way despite near-unanimous Welsh opposition at every political level – considerably boosted support for Plaid Cymru, the Welsh nationalist party*.

In October 2005, forty years after the official opening, Liverpool City Council formally apologised for the flooding.

Arenig Fach (689 m asl) is visible at the western end of the lake, whilst Arenig Fawr (854 m) rises from the southern bank, off the left of the photo.


*: I'd better take this opportunity to stress that Plaid is "the party for the people of Wales, representing everyone who has chosen to make Wales their home". It's not an 'English-out' racist organisation and is utterly different to the xenophobic BNP.