Complex sentence: Capel Celyn was a rural community to the north west of Bala in north Wales, in the Afon Tryweryn valley that was flooded to create a reservoir, Llyn Celyn, to supply Liverpool and The Wirral, with water for industry.
Split into simpler sentences: Capel Celyn was a rural community to the north west of Bala in north Wales, in the Afon Tryweryn valley. The village and other parts of the valley were flooded to create a reservoir, Llyn Celyn, in order to supply Liverpool and The Wirral with water for industry.