Like Llyn Trawsfynydd, Tanygrisiau Reservoir (more accurately called Llyn Ystradau) was constructed to produce electricity, this time by harnessing the power from Llyn Stwlan, located another thousand feet higher up in the towering Moelwyn Mountains. Construction of the Ffestiniog Power Station started in 1957 and was completed in 1963.
Water is pumped from the lower to the upper reservoir at times of low demand (usually at night) and the water is then released through pipes to power its four generators. The resulting electricity is then provided to the National Grid at times of high demand. During this time there is a 5.5 meter gain between the low and high water levels.
Constructed in 1960, the main victim of the flooding was the Ffestiniog Railway which ran along the once a boggy valley after exiting its original 660-meter long tunnel. As the route was blocked, a spiral loop was built at Dduallt so that trains could gain enough height to run along a new 262-meter tunnel, exiting above the lake and running along the side of the mountain. This was all completed in 1977 and is the only loop of its kind in Britain.