In 1960, a private bill sponsored by Liverpool City Council was brought before Parliament to develop a water reservoir in the Tryweryn Valley. The development would include the flooding of Capel Celyn. By obtaining authority via an Act of Parliament, Liverpool City Council would not require planning consent from the relevant Welsh local authorities and would also avoid a planning enquiry at Welsh level at which arguments against the proposal could be expressed. This, together with the fact that the village was one of the last Welsh-only speaking communities, ensured that the proposals became deeply controversial. Thirty-five out of thirty-six Welsh Members of Parliament (MPs) opposed the bill (the other did not vote), but in 1962 it was passed. The members of the community waged an eight-year effort, ultimately unsuccessful, to prevent the destruction of their homes.
When the valley was flooded in 1965, the village and its buildings, including the post office, the school, and a chapel with cemetery, were all lost. Twelve houses and farms were submerged, and 48 people of the 67 who lived in the valley lost their homes.[2] In all, some 800 acres (3.2 km²; 320 ha) of land were submerged. A new reservoir, Llyn Celyn, was formed. Many of the stones from the original chapel were re-used in the construction of the new Memorial Chapel.
The water in the reservoir is used to maintain the flow of the River Dee (Afon Dyfrdwy) so that water may be abstracted downstream,[3] and additionally to improve the quality of white-water sports on Afon Tryweryn.