Llyn Trawsfynydd

Infobox lake
lake_name = Llyn Trawsfynydd
image_lake = Llyn Trawsfynydd.jpg
caption_lake = looking south
image_bathymetry =
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location = Gwynedd, North Wales
coords = coord|52|54|40|N|3|56|40|W|region:GB_type:waterbody|display=inline,title
type = reservoir
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basin_countries = United Kingdom
length =
width =
area = convert|1180|acre|km2|abbr=on|lk=on
depth =
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Llyn Trawsfynydd is a large man-made reservoir located near the village of Trawsfynydd in Gwynedd, North Wales. It covers an area of 1,180 acres and as such is larger than Wales' largest natural lake, Llyn Tegid.

It was originally built to supply water for Maentwrog hydro-electric power station in 1928, and was subsequently also used to supply cooling water to Trawsfynydd's twin reactor nuclear power station, used for the commercial generation of electricity for the UK National Grid.

The Nuclear power station no longer generates electricity, and is in the process of decommissioning but the lake remains in use as the header reservoir for hydroelectricity generation.