The River Tay, in terms of flow, is the largest river in Scotland, and drains much of the southern Highlands. The source of the Tay is high on the slopes of Ben Lui, only 20 or so miles from the west coast town of Oban, but the waters flow through Perthshire to the North Sea, some 100 miles east. The river has a variety of names in the upper catchment: for the first few miles the river is known as the River Connonish; then it is called the River Fillan, and then the name changes again 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 where it becomes tidal and enters the Firth of Tay. The main tributaries of the River Tay are the Almond, Isla, Braan, Tummel and Lyon. Like the River Spey, River Dee and River Tweed, the River Tay is a famous salmon river.
The catchment of the Tay system covers an area of 4970 square kilometres. The average annual flow of the River Tay at Perth is about 170 cubic metres per second (m3 s-1). The maximum recorded flow of 2269 m3 s-1 was recorded on [January 17 1993, when the river rose 6.48 m above its usual level at Perth, and caused extensive flooding in the city. Were it not for the hydro-electric schemes upstream which impounded run-off, the peak would have been considerably higher. The highest ever flood at Perth occurred in 1814, when the river rose 7 m above the usual level, partly caused by a blockage of ice under the Smeaton Bridge. Other severe flood events occurred in 1210 and 1648 when earlier bridges over the Tay at Perth were destroyed.