Water, depending on where it originates and the path it takes, may pass through as many as five of the schemes nine power stations as it progresses from north-west to south-east.

The promotors were aware of the "fundamental principles" set out by the Snell Committee, and ensured that these formed part of the bill.

The promotors included the Duke of Atholl and the chairman of Lloyds Bank, John William Beaumont Pease, both men who were known to be honest and trustworthy, and who were held in high regard locally.

Power Securities was created specifically to provide funding for capital intensive schemes, and despite the deep depression of late 1922, were able to help.

They realised that the scheme could only be economic if some of the power generated was exported to the industrial heartland of central Scotland, as their own area was too sparsely populated.

Indeed, the newly-formed Central Electricity Board rejected the idea of extending the national grid into the Grampian area, because of its low demand for power.

The tunnel was nearly 3 miles (4.8 km) long, and a steel pipeline then ran down the hillside to Rannoch power station.

Several smaller projects supplemented the flow into Loch Ericht by diverting the headwaters of the River Truim, the Allt Ghlas, and other streams.

At its southern end, the dam was built on the River Ericht some distance downstream of the loch's outlet, which raised the water level by 14 feet (4.3 m).

Steel towers, some 97 feet (30 m) high, carried 132 kV power lines east along the bank of the Loch, continuing for 58 miles (93 km) to connect to the National Grid at Abernethy, to the south-east of Perth.

[13] Loch Rannoch formed the main storage for the next station in line at Tummel, about 8 miles (13 km) downstream to the east.

The weir at Kinloch Rannoch regulates the depth at the exit of Loch Rannoch to 8 feet (2.4 m), this being an increase of about 4 ft (1.2 m), with water passing down the River Tummel for 5 miles (8 km) to a narrow point where Dunalastair dam creates an artificial loch known as Dunalastair Water.

[14] Subsequently, both Tummel and Rannoch power stations have become Category A listed buildings, being designed in "a simple classical style and with a bold outline.

"[15][11] Kinloch Rannoch weir consists of three floodgates, each 40 feet (12.2 m) wide, which were designed and built by Glenfield and Kennedy of Kilmarnock, Ayrshire.

The gates lasted for almost 80 years, but in 2009, SSE decided that they needed to be replaced, and a contract for the work was awarded to Site Services of Aberdeen.

The weight limit allowed them to be lifted into place by an existing overhead gantry, as it would have been too difficult to access the site with a mobile crane.

Johnston therefore set up a tribunal to consider the application, overseen by John Cameron, Sir Robert Bryce Walker and Major G H M Brown Lindsay.

[21] Edward MacColl, the Board's Chief Executive, was unable to appear as he was ill, and Lord Airlie was a less able witness, who had suffered months of vicious abuse in the press.

[22] From day six, fishery experts, civil engineers, architects, hotel owners, youth hostelers and environmentalists argued the case for the opposition.

[23] The allegations were acrimonious, but R P Morison, summing up for the Board at the end of the proceedings, stated that the benefits of providing electricity to remote areas where it would be uneconomic to do so without such a scheme outweighed the small amount of damage that might be done, and he was adamant that the tourists would still come to Pitlochry and that the livelihoods of the residents would be maintained.

[24] There were attempts to derail the progress of the bill in Parliament, which raised the same issues again, but the MP for Perth and Kinross, William Snadden, failed to convince the House of Commons that the confirmation order should be annulled, and Lord Kinnaird was persuaded not to bring a similar motion in the House of Lords.

and is described by Historic Environment Scotland as "an outstanding example...a bold modernist design by Harold Tarbolton.

[27] The contracting engineers were Sir Alexander Gibb and Partners and around 12,000 workers were employed at the peak of construction with workers including German and Italian former prisoners of war as well as a cohort of Donegal Tunnel Tigers, Irish tunnelling specialists who moved from job to job.

A memorial arch, with the same cross section as the tunnel, was built at the entrance to the car park, to commemorate those who died during the construction.

[39][40] There are four main water paths in the completed scheme, all of which ultimately discharge into Loch Tummel to feed the Clunie and Pitlochry stations.

From the north the highest storage point in the scheme is at Loch an t-Seilich, which is at 1,391 ft (424 m) above ordnance datum (AOD).

[41] Water passes through a tunnel to Loch Cuaich at 1,319 feet (402 m) AOD, which covers 115 acres (47 ha) and has a catchment area of 5.97 square miles (15.47 km2).

The construction of this dam raised the water level by 17 feet (5.2 m)[49] to create a reservoir with a capacity of 36,400,000 cubic metres.

The scheme power stations are generally, though not exclusively, named for their physical location as opposed to the source of their water supply.

)[53] Near the highest point in the scheme the upper River Garry was completely dry for a distance of 20 km (12 miles) from the 1950s until a project in 2017 returned some water, around 1 m3 (35 cu ft) per second,[54] to the section.