Forth, river, c.60 mi (100 km) long, formed by streams that join near Aberfoyle in Stirling, S central Scotland. It meanders generally eastward past the town of Stirling to the Firth of Forth at Alloa. Its chief tributaries are the Teith and Allan rivers. The Firth of Forth extends c.55 mi (90 km) E from Alloa to the North Sea, reaching widths up to 19 mi (31 km) across. Rosyth is an important naval base, and Leith is the port of Edinburgh. The port of Grangemouth is at the eastern end of the Forth and Clyde Canal (35 mi/56 km long; completed 1890), which links the Firth of Forth with the River Clyde. Rivers flowing into the firth include the Leven, Esk, Avon, and Carron. The Isle of May and Bass Rock, with lighthouses and ruins, are at the entrance to the firth; Inchkeith and Inchcolm islands are within the firth. At Queensferry three bridges cross the firth: the Forth Bridge, also known as Forth Railway Bridge (5,350 ft/1,631 m; completed 1890), the world's first cantilever bridge; the Forth Road Bridge, one of the longest suspension bridges in Europe (3,300 ft/1,006 m long; completed 1964); and the Queensferry Crossing, a cable-stayed road bridge (8,858 ft/2,700 m long; completed 2017). Two more bridges cross the firth farther inland, at or near Kincardine.

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