This is a historical note about the story behind the phrase, Cofiwch Dryweryn.

In July 1957, the British Parliament approved a plan by the Liverpool City Council to flood the Tryweryn Valley, located northwest of Bala in Gwynedd for water for Liverpool and the Wirral, primarily for industry. They came to Wales, not because water was unavailable elsewhere, but because they could get it at a lower cost.

The act that was passed was a ‘private’ one and thus done without the consent of Welsh authorities and, in fact, against the wishes of all but one Welsh Member of Parliament. The local councils and local population objected formally to the idea, but despite their protests, the Liverpool council went ahead with their plan and flooded the valley in 1965. Many communities, including Capel Celyn, were lost and their inhabitants forcibly removed. The chapel and other buildings in the village were repurposed for the construction of a memorial chapel. It was completed in 1967 and overlooks the reservoir at the north-west end. Families who had relatives buried in the cemetery were given the option of moving them, though in the end only eight bodies were disinterred. All the headstones were moved to the memorial chapel.

At the time, Wales had no Welsh office or any devolution of powers from the national government in London. The flooding of Tryweryn is viewed as a pivotal moment for Welsh nationalism and led directly to the formation of the Welsh nationalist political party, Plaid Cymry, and the election of its first member of Parliament in 1966.

Cofiwch Tryweryn, meaning “Remember Tryweryn”, was painted by Meic Stephens for the first time on the wall of a ruined cottage near Llanrhystud, Ceredigion in 1962. Because it was grammatically incorrect, later repaintings corrected the spelling to Cofiwch Dryweryn.

Arguably, the flooding of the Tryweryn Valley led ultimately to the devolution of powers and the creation of the Welsh National Assembly in 1999. Even then, however, the people of Wales still have little say over their natural resources, which remain outside the remit of the National Assembly, with the Secretary of State reserving the right to intervene on any Welsh legislation that (quote) “might have a serious adverse impact on water resources in England, water supply in England or the quality of water in England”.
In recent years, Cofiwch Dryweryn has been painted in more than one hundred and fifty other places throughout Wales and has become a rallying cry for Welsh autonomy.

The Tryweryn Valley was renamed Llyn Celyn (Celyn Lake) in 1964 by the Liverpool Corporation, now United Utilities, which still owns it in order to distribute water to Liverpool.