Firth is the Lowland Scots word used to denote various coastal waters in Scotland. It is usually a large sea bay, which may be part of an estuary, or just an inlet, or even a strait. It is cognate to fjord (both from Proto-Germanic *ferþuz) which has a more narrow sense in English, whereas a firth would most likely be called a fjord if it were situated in Scandinavia. Bodies of water named "firths" tend to be more common on the east coast, or in the southwest of the country, although the Firth of Lorn is an exception to this. The Highland coast contains numerous estuaries, straits and inlets of a similar kind, not called "firth", e.g. the Minch, and Loch Torridon; these are often called sea lochs.
A firth is generally the result of ice age glaciation and is 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. Demarcation can be rather vague. The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow, while locally the change is held to be at the Tail of the Bank where the river crosses a sandbar off Greenock at the junction to the Gare Loch, or even further west at Gourock point.
However, some firths are exceptions. 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. The Pentland Firth is a strait rather than a bay or an inlet.
Firths on the west coast of Scotland (from north to south)
- Firth of Lorn (northernmost, connects with the Moray Firth via the
- Firth of Clyde (continuing from the River Clyde estuary)
- Sea lochs adjoining the Firth of Clyde: Gare Loch, Loch Long, Holy Loch, Loch Striven, Loch Riddon off the Kyles of Bute, Loch Fyne and Campbeltown Loch.
- Places: Helensburgh, Port Glasgow, Greenock, Gourock, Dunoon, Rothesay, Wemyss Bay, Largs, Brodick, Ardrossan, Troon, Ayr, Girvan and Campbeltown. Note that Glasgow is at the tidal limit of the River Clyde, and Clydebank, the Erskine Bridge and Dumbarton are on the river estuary as it widens out towards Port Glasgow.
- Islands: Bute, Cumbrae, Arran
- :In Scottish Gaelic, the Firth of Clyde is treated as two bodies, with the landward end being called Linne Chluaidh (meaning the same as the English), while the area around the south of Arran, Kintyre and Ayrshire/Galloway is An Linne Ghlas ().
- Solway Firth (inlet with the rivers Eden, Esk and Nith).
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)
- Cromarty Firth (loch-type firth with relatively narrow opening to the sea). The Firth runs out into the Moray Firth.
- Moray Firth and Beauly Firth (a loch-type firth) connected with the Firth of Inverness. The Firth of Inverness is rarely identified on modern maps, but 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 Lorne on the west coast of Scotland.
- Firth of Tay (estuary of the River Tay).
- 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 Forth Road Bridge, 2,512 m (8242 ft) long, and the Forth Bridge (the adjacent railway bridge), 2,498m (8,196ft) long.
- Rivers: Forth, River Avon, Water of Leith, River Almond, River Esk, River Leven
- Islands: Bass Rock, Craigleith, Eyebroughy, Fidra, Inchcolm, Inchgarvie, Inchkeith, Inchmickery, Isle of May, The Lamb
Firths on the north coast of Scotland
- The Pentland Firth. This is a strait between the Scottish mainland and the Orkney Islands, and forms a link between the Atlantic Ocean and North Sea.
- Places: John o' Groats, Canisbay, Gills Bay, Rattar (all Caithness)
- Headlands: Brims Ness, Brough Ness, Duncansby Head, Dunnet Head
- Islands: Hoy, Pentland Skerries, Swona, South Ronaldsay, South Walls (all generally considered to be part of Orkney); Stroma
The Northern Isles were part of Norway until the 15th century, and retain many Norse names. In Shetland in particular, "firth" can refer to smaller inlets, although geo, voe and wick are as common. In Orkney, "wick" is common.
Other similar waters in Scotland
In the Scottish Gaelic language, linne is used to refer to most of the firths above; it is also applied to the Sound of Sleat, Crowlin Sound, Cuillin Sound, Sound of Jura, Sound of Raasay, and part of Loch Linnhe.
The following is a selection of other bodies of water in Scotland which are similar to various firths, but which are not termed such -
Likewise, in the Northern Isles, the words "firth" and "sound" are often used arbitrarily or interchangeably. Bluemull Sound for example, is very similar to some of the firths in the Shetland Islands.
Firths outside Scottish waters