The Firth of Clyde is the estuary of the River Clyde, on the west coast of Scotland. The Firth has some of the deepest coastal waters of the British Isles and is sheltered from the Atlantic Ocean by the Kintyre Peninsula. The Firth lies between West Dunbartonshire in the north, Argyll and Bute in the west, and Inverclyde, North Ayrshire and South Ayrshire in the east. Kilbrannan Sound is a large arm of the Firth, separating the Kintyre Peninsula from the Isle of Arran. The Kyles of Bute separates the Isle of Bute from the Cowal Peninsula. The Sound of Bute separates the islands of Bute and Arran.
The Highland Boundary Fault crosses the Firth. The Firth also played a vital military role during World War II.
The Firth is sometimes called the Clyde Waters or Clyde Sea. It is customarily considered to be part of the Irish Sea.
Geography
thumb|The [[West Kilbride|Seamill beach looks south down the outer firth towards southern Arran and Ailsa Craig]]
thumb|Firth of Clyde View
thumb|Gourock and the Firth of Clyde
At the north of the Firth, Loch Long and the Gare Loch join the Firth; these lochs are separated by the Rosneath Peninsula. Off Greenock, an anchorage known as the Tail of the Bank narrows the estuary of the River Clyde to wide. (The "Bank" is a reference to a sandbank and shoal) The River Clyde estuary has an upper tidal limit located at the tidal weir next to Glasgow Green.
The geographical (and popular) distinction between the Firth and the River Clyde is vague. Some refer to Dumbarton as being "on the Firth of Clyde"; while at the same time, the residents of Port Glasgow and Greenock often refer to the part of the Firth that lies to the north of those areas as "the river".
The Firth encompasses many islands and peninsulas. Twelve ferry routes connect them to each other and the mainland. The majority of the ferry services are run by Caledonian MacBrayne and one by Western Ferries, and many of the routes are lifeline services for communities living in remote areas. The Firth has no fixed link transport infrastructure connections (bridge or tunnel), linking the shores or islands. The lowest fixed crossing over the Clyde is the Erskine Bridge, opened on 2 July 1971.
There are two canal connections from the Firth. The Forth and Clyde Canal at Bowling, West Dunbartonshire and the Crinan Canal at Ardrishaig.
The Irish Sea and the Firth of Clyde's southerly boundary, as defined by the Scottish Government, is between the southern tip of the Kintyre Peninsula and Corsewall Point on the Rhins of Galloway.
Highland Boundary Fault
The Highland Boundary Fault enters the Firth off the east coast of Kintyre Peninsula in the south. The fault crosses the south eastern tip of the Cowal Peninsula at Toward Point, where it can be seen on the surface by the presence of Old Red Sandstone. The fault continues to Helensburgh in the north, then continues past the east coast of Scotland. The fault can be followed across Scotland for at least . The fault is of great age and its remains are broken by more recent geological movement of the earth's crust.
Sea Lochs
Fourteen sea lochs join the Firth, the largest being Loch Fyne.
Peninsulas
- Cowal Peninsula
The Cowal Peninsula extends into the Firth of Clyde and forms the main western shoreline of the upper Firth. The main town on the Cowal Peninsula is Dunoon. Ardlamont Point on the Ardlamont Peninsula, which extends off the Cowal Peninsula, is the southern tip of the Ardlamont and Cowal Peninsulas.
The ferries across the Firth save time compared to traveling "round by road", via Loch Eck side (A815 road), the Rest and Be Thankful (A83 road) and Loch Lomond side (A82 road).The service between Dunoon and Gourock in Inverclyde is operated by Caledonian MacBrayne, the Public Service provider. This service carries only foot passengers and connects directly with the ScotRail service to Glasgow.Western Ferries is a Private Limited Company; it operates the service between Hunters Quay and McInroy's Point near to Gourock. This service carries all types of vehicular transport, as well as foot passengers.
- Kintyre Peninsula
The Kintyre peninsula forms the main west coastline of the lower Firth.
- Rosneath Peninsula
The Rosneath peninsula is formed by the Gare Loch in the east, and Loch Long in the west; both merge with the upper Firth of Clyde. There is a Caledonian MacBrayne passenger-only service across the Firth to Gourock from Kilcreggan.
Firth Islands
thumb|Arran sunset
There are around forty islands and skerries in the Firth. The largest three all have thriving communities and have ferry services connecting them to the mainland. They are:
- Arran
- Bute
- Great Cumbrae
Ailsa Craig is a small island, formed by the remains of a volcanic plug in the Firth. Ailsa Craig Common Green Granite and Ailsa Craig Blue Hone Granite are quarried there for the production of Curling Stones, manufactured by Kays of Scotland.
Holy Isle, a small island lying in Lamlash Bay, Isle of Arran. At the north of the Holy Isle, there is a religious resort.
Firth Lighthouses
The Northern Lighthouse Board is responsible for some of the navigation aids around Scotland's coast. Others are the responsibility of the local Competent Harbour Authority.
thumb|The Gantocks with Cloch Lighthouse behind
There are lighthouses at:
There are navigation beacons at:
- The Gantocks Rocks and Navigation Beacon, off the coast at Dunoon, Cowal Peninsula.
- Horse Isle Beacon, Horse Isle, Ardrossan Built 1811.