Forth, a river and firth of Scotland. The river is formed by two head-streams, Duchray Water and the Avondhu, which, rising on and not far from Ben Lomond, at altitudes of 3000 and 1900 feet, run 14 and 9 miles to a confluence near Aberfoyle, the Avondhu traversing Lochs Chon and Ard. From their confluence, 80 feet above sea-level, the Forth itself winds 39 miles to Stirling, then 12½ (the 'Links of Forth') to Alloa, the distances in a straight line being only 18½ and 5¾ miles. It receives the Teith, Allan Water, and Devon, and traverses or divides Stirling, Perth, and Clackmannan shires.
The Firth of Forth extends 51 miles eastward from Alloa to the German Ocean, between the counties of Clackmannan and Fife on the north, and Stirlingshire and the Lothians on the south. It has a width of ½ mile at Kincardine, 3 miles above Bo'ness, 1¼ at Queensferry, 5 between Granton and Burntisland, 17 at Prestonpans, and 8¼ at Elie. Its waters, 3 to 37 fathoms deep, encircle the islands of Inchkeith (fortified 1878-81), Inchcolm (with a ruined abbey), Cramond, &c., whilst at the entrance are the Bass Rock (q.v.) and the Isle of May, on which last and on Inchkeith are lighthouses. Rivers falling into it are the Carron, Avon, Almond, Water of Leith, Esk, and Leven.
White fish are plentiful. In 1882-90 a great railway bridge was erected across the firth at Queensferry (see BRIDGE), above which is St Margaret's Hope, one of the safest roadsteads in the kingdom.