While Llyn Trawsfynydd is 67 acres bigger than Llyn Tegid, that is Wales’ biggest natural lake, while this is Wales’ largest man-made one. It was built to supply water for the Maentwrog hydro-electric power station which was built at the same time.
The whole lake has four dams, one of the main rivers that were dammed being the Prysor. The whole process took four years to build and was completed in 1928 and, when completed, it provided enough electricity for the whole of North Wales.
The lake was drained in 1965 when the Trawsfynydd nuclear power station was built (Britain’s first to be built inland). The deep water of the lake was used as the source of the cooling water needed, although another dam was needed to increase the volume of water. As the water needed to be cold, it needed to be circulated around the whole lake, a process which took between five and eight days. The Maentwrog station was still in use and was using the warmer water nearer the surface.
The lake was used until the nuclear power station was closed and decommissioning work started in 1991, a process that won’t be completed until around 2083. After closure, the water temperature has returned to natural levels which has regenerated some of the plant and fish life.
It is now a popular location for fishing, walking and cycling.