Firth is the Scots word often used to denote a large sea bay in Scotland, which may be part of an estuary, or just an inlet, or even a strait (as in the case of the Pentland Firth). It is cognate to fjord, which has a more narrow sense in English, whereas a firth would most likely be called a fjord if it were situated in Scandinavia.

A firth is generally the result of glaciation and very often associated with a large river, where erosion caused by the tidal effects of incoming sea water passing upriver has widened the riverbed to an estuary, such as may be seen in the Firth of Clyde. However, this cannot be said in every case. The Cromarty Firth on the East coast of Scotland, for example, resembles a large loch with only a relatively small outlet to the sea and the Solway Firth and the Moray Firth are more like extremely large bays.

See Pentland Firth.

The Firths on the West Coast of Scotland from North to South

• Firth of Lorne (northernmost, connects with the Moray Firth via the
  • Great Glen lochs, the Caledonian Canal and Loch Ness at Inverness.
  • Lochs adjoining the Firth: Loch Lochy, Loch Linnhe, Loch Leven, Loch Oich.
  • Places: Oban, Fort William. Islands: Island of Mull, Lismore and Kerrera.

• Firth of Clyde (estuary of the River Clyde)
  • Sea lochs adjoining the Firth of Clyde: Loch Goit, Loch Eck, Loch Long, Holy Loch and Gare Loch.
  • Places: Erskine Bridge, Greenock, Clydebank, Glasgow, Dunoon, Helensburgh. Dumbarton.
  • Islands: Bute, Great Cumbrae, Islands of the lower Firth of Clyde.

• Solway Firth (inlet with the rivers Eden, Esk and Nith).
  • The Firth is off the Solway Coast.
  • Places: Carlisle, England on the River Eden, Annan and Gretna, both in Scotland.

The Firths on the East coast of Scotland from North to South (these are connected to, or form part of, the North Sea)

• Dornoch Firth (northernmost of the Eastern firths)
  • Places: Dornoch, Dornoch Bridge (impressive road bridge, half a mile long), Bonar Bridge, Kyle of Sutherland, Tain, Portmahomack on Tarbat Ness (fishing village facing West to Northwest on the East coast).
  • River: Shin.
  • Headland: Tarbat Ness.

• Cromarty Firth (loch type firth with relatively narrow opening to the sea). The Firth runs out into the Moray Firth.
  • Places: Cromarty, Dingwall, Invergordon.
  • Rivers: Conon, Orrin, Rusdale, Glass, Alness.

• Moray Firth and Beauly Firth (two loch-type firths connected with each other with Firth of Inverness between the two). The Firth of Inverness is rarely identified on modern maps, but it is this firth which forms a connection via the River Ness, Loch Ness and the other lochs of the Great Glen and stretches of the Caledonian Canal with the Firth of Lorn on the West coast of Scotland.
  • Places on the Moray Firth: Inverness, Nairn, Fortrose, Fort George.
  • Headlands: Whiteness Head, Chanonry Point, Alturlie Point.
  • Places on the Beauly Firth: Beauly.

• Firth of Tay (estuary of the River Tay).
  • Places: Perth, Dundee, Monifieth, Tayport, Newport on Tay.
  • Rivers: Tay, Earn.
  • Headland: Buddon Ness.

• Firth of Forth (estuary of the River Forth)
  • Places: Edinburgh, Dunfermline, Kirkcaldy, Falkirk, Stirling, Grangemouth, Rosyth, North Queensferry, South Queensferry, Musselburgh, Crail, Cellardyke, Anstruther, Pittenweem, St Monans, Elie, Earlsferry. It is spanned by the magnificent Firth of Forth Road Bridge, 1,006m (3,300ft) long, and the Forth Rail Bridge, 2.498m (8,196ft) long.

Firths outside Scottish waters

• The Firth of Thames is a bay at the mouth of the Waihou/Thames River in New Zealand.

See also: List of waterways