The scheme harnessed the headwaters of the Rivers Treig and Spean and the floodwaters of the River Spey (plus a further eleven burns along the way). The Laggan Dam (213 m long and 55 m high) contained the flow of the Spean in a reservoir. A 4 km tunnel then linked this body of water with another reservoir (Loch Treig) contained by the Treig dam. From here, the main tunnel, until 1970 the longest water-carrying tunnel in the world, at 24 km (15 miles) long and 5 m in diameter, was driven around the Ben Nevis massif. From the western mountainside, down five massive steel penstocks, the water was channelled to the turbines in the power house at the smelting plant.