Forth, a river of Scotland, the third of that country in size, and one of the most noted for romantic scenery. It is formed by the confluence of two small streams, the Duchray and the Dhu, which unite on the X. E. slope of Ben Lomond. Thence, under the name of the Avendow or Black river, it flows E. through the fertile valley of the Laggan, shut in on either side by hills, and after receiving one or two tributaries assumes the name of Forth. From this point it begins to present the remarkable sinuosities which form its chief characteristic, now winding gracefully through a rich level country, now doubling and flowingW., again sweeping to the E., describing at times almost complete circles, and forming all along its course many beautiful peninsulas. The most notable of these windings, called the "links of Forth," occur between Alloa and Stirling, the distance between which places, in a straight line, is about 6 m., while by water it is 12 m. The Teith, Allan, and Devon are its largest tributaries. At Kincardine it begins to widen into an estuary, called the frith of Forth, between the counties of Clackmannan and Fife on the north, and of Linlithgow, Edinburgh, and Haddington on the south.
The frith contains several islands, and a great abundance of herring and other fish; length 50 m., greatest breadth 15 m. The general course of the Forth is E. or S. E. Its depth is from 3 to more than 37 fathoms, and its bottom is generally muddy. The tide sets up from the sea as far as Stirling bridge, a distance of 70 m. It is navigable thus far for vessels of 100 tons, and to Alloa for vessels of 300 tons. Its length to the sea, including all its sinuosities, is about 170 m., though in a direct line it would not exceed 90 m. The Forth and Clyde canal, 38 m. in length, connects those two rivers.