English: North Wales. Llyn Peris as seen from the Dinorwig to Nan Peris slate trail. Llyn Paris is used as the lower reservoir for the Hydro-electric power station built into the mountainside. Water from an upper reservoir (Marchyn Mawr) generates electricity during daytime peaks but at night when the demand lessens the same turbines pump the water up to upper reservoir for use the next day. You are looking down at the outlet from the mountain into Llyn Peris.