River Lyon, River Tummel, Isla Almond, Earn, River Braan
Source: Allt Coire Laoigh
- location Ben Lui, Scottish Highlands, UK - elevation 720 m (2,362 ft) - coordinates 56°23′07″N 4°47′36″W
Mouth: Firth of Tay, North Sea - location Between Perth, Scotland and Dundee, Scotland, UK - elevation 0 m (0 ft) - coordinates 56°21′18″N 3°17′54″W
Length: 188 km (117 mi)
Basin: 4,970 km2 (1,919 sq mi)
Discharge for Ballathie
- average 169 m3/s (5,968 cu ft/s) - max 1,965 m3/s (69,393 cu ft/s) - min 11 m3/s (388 cu ft/s)
The River Tay (Gaelic: Tatha) 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 (Beinn Laoigh), 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 volume of discharge.<ref>http://www.peer.eu/fileadmin/user_upload/projects/flagship_projects/PEER_Euraqua/Tay%20UK.pdf (accessed 7th July 2014)</ref> Its catchment is approximately 2,000 square miles (5,200 km2), the Tweed's is 1,500 square miles (3,900 km2) and the Spey's is 1,097 square miles (2,840 km2).
The Tay drains much of the lower region of the Highlands. It originates on the slopes of Ben Lui (Beinn Laoigh), only c. 30 miles (c. 32 km) from the west coast town of Oban, in Argyll and Bute.<ref>http://www.peer.eu/fileadmin/user_upload/projects/flagship_projects/PEER_Euraqua/Tay%20UK.pdf (accessed 7th July 2014)</ref>
In 2011, the Tay Western Catchments Partnership determined as its source (as based on its 'most dominant and longest' tributary) a small lochan on Allt Coire Laoigh south of the summit.<ref>http://www.bbc.co.uk/news/uk-scotland-tayside-central-12243508 (accessed 7th July 2014)</ref>The river has a variety of names in its upper catchment: for the first few miles it is known as the River Connonish; then the River Fillan; the name then changes to the River Dochart until it flows into Loch Tay at Killin.
The River Tay emerges from Loch Tay at Kenmore, and flows from there to Perth which, in historical times, was its lowest bridging point. Below Perth the river becomes tidal and enters the Firth of Tay. The largest city on the river, Dundee, lies on the north bank of the Firth. On reaching the North Sea, the River Tay has flowed 120 miles (193 km) from west to east across central Scotland.
The Tay is unusual amongst Scottish rivers in having several major tributaries, notably the Earn, the Isla, the River Tummel, the Almond and the Lyon.<ref>http://www.peer.eu/fileadmin/user_upload/projects/flagship_projects/PEER_Euraqua/Tay%20UK.pdf (accessed 7th July 2014)</ref>
A flow of 2269 m3/s was recorded on 17 January 1993, when the river rose 6.48 metres above its usual level at Perth, and caused extensive flooding in the city. Were it not for the hydro-electric schemes upstream which impounded runoff, the peak would have been considerably higher. The highest flood recorded at Perth occurred in 1814, when the river rose 7 m above its usual level, partly caused by a blockage of ice under Smeaton's Bridge. Other severe flood events occurred in 1210 and 1648 when bridges over the Tay at Perth were destroyed.