The abundant rainfall and the favourable topographical and geological conditions make the west of Scotland an ideal site for the production of cheap power, for the manufacture of aluminium.
In 1921, after many years of study and debate, Parliament sanctioned the ambitious project to build a new aluminium factory with its own hydroelectric installation harnessing the power of the Rivers Treig and Spean and the flood waters of the River Spey.
The construction was carried out in three stages extending over many years. Stage 1 involved feeding the waters through a concrete lined pressure tunnel and through steel penstocks to a power station. The main tunnel, 5m in diameter for 24km, was driven through the massif centred on Ben Nevis and was ranked the longest water carrying tunnel in the world at the time.
In the second stage essential storage was provided by the construction of the Laggan Dam, an arch-gravity dam 213m long and 55m high on the River Spean.
From this reservoir the water is conveyed through a 4.5, diametre tunnel to Loch Treig, the level of which was raised 11m by building Treig Dam, a rock fill dam with an impervious concrete core. From Lock Treig the water is conveyed to the Power House with atotal output of 72 megawatts.
The third and last stage of development covered work on a composite dam 378m long and 18m high across the River Spey.
The purpose of this dam was to divert the flood waters of the Spey through a conduit and yet another tunnel which was also constructed to feed water into the eastern end of the Loch Laggan reservoir.
At the peak of construction over 3000 men were employed on the Lochaber project which eventually provided a total installed capacity of 88 megawatts.