River Tay River Tay The River Tay () is the longest river in Scotland and the seventh-longest in the United Kingdom. The Tay originates in western Scotland on the slopes of Ben Lui (), then flows easterly across the Highlands, through Loch Dochart, Loch Iubhair and Loch Tay, then continues east through Strathtay (see Strath), in the centre of Scotland, then southeasterly through Perth, where it becomes tidal, to its mouth at the Firth of Tay, south of Dundee. It is the largest river in the UK by measured discharge. Its catchment is approximately , the Tweed's is and the Spey's is
Tay Rail Bridge Bouch had used the technique for viaducts including the Belah Viaduct (1860)) on the South Durham & Lancashire Union Railway line over Stainmore, but for the Tay Bridge, even with the largest practicable caissons the pier dimensions were constrained by their size. Bouch’s pier design set six columns in a hexagon maximising the pier width but not the number of diagonal braces directly resisting sideways forces. The engineering details on the Tay Bridge were considerably simpler, lighter, and cheaper than on the earlier viaducts. On these the machined base of each column section docked securely into a machined enlarged section