sentences:  This Formation also reappears in Afon Tryweryn to the south-east, where the hard rock was utilized in the construction of Llyn Celyn dam.Llyn Conwy is owned by the National Trust and, whilst a natural lake, is managed as a reservoir by Welsh Water.It supplies some of the water needs of the upper Conwy Valley, including Betws-y-coed and Llanrwst.