Kyle of Sutherland is a river estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

 that separates Sutherland

Sutherland

Sutherland is a registration county, lieutenancy area and historic administrative county of Scotland. It is now within the Highland local government area. In Gaelic the area is referred to according to its traditional areas: Dùthaich 'IcAoidh , Asainte , and Cataibh...

 from Ross-shire

Ross-shire

Ross-shire is an area in the Highland Council Area in Scotland. The name is now used as a geographic or cultural term, equivalent to Ross. Until 1889 the term denoted a county of Scotland, also known as the County of Ross...

. It flows into the Dornoch Firth

Dornoch Firth

The Dornoch Firth is a firth on the east coast of Highland, in northern Scotland. It forms part of the boundary between Ross and Cromarty, to the south, and Sutherland, to the north....

 and is fed by the rivers Oykel

River Oykel

The River Oykel is a major river in northern Scotland that is famous for its salmon fishing. It rises on Ben More Assynt, a few miles from Ullapool on the west coast of Scotland, and drains into the North Sea via the Kyle of Sutherland...

, Shin

River Shin

The River Shin is a river in the Scottish North West Highlands.The river flows from Loch Shin into the Dornoch Firth and then into the North Sea. The river is just long....

, River Cassley

River Cassley

The River Cassley in Sutherland, in northern Scotland, drains into the Kyle of Sutherland at Invercassley , a short distance below where the River Oykel flows into the Kyle. The Kyle is subsequently joined by the River Shin about Inveran, before it becomes the Dornoch Firth at Bonar Bridge, into...

 and Carron

River Carron, Sutherland

The River Carron is a river in Sutherland, in the Highlands of Scotland.It begins as the Abhainn a' Ghlinne Mhoir, which joins with the Alladale River to form the River Carron...

.

The downstream extent of the Kyle of Sutherland is the bridge at Bonar Bridge. The upstream end of 'the Kyle' as it is locally known is the furthest inland extent of tidal water, which corresponds to 'the bailey bridge', beyond Rosehall.

The Kyle did separate Sutherland-shire and Ross-shire for centuries until 1975 when The Highland Region reorganised its internal boundaries leaving both banks within Sutherland.