The river has it origin in Loch Sealga, in a narrow glen south of the towering peaks of An Teallach in Ross-shire. The Srath na Sealga River is the principal feeder for the loch, but the river which emerges at the loch's foot is the Gruinard.
After a course of several miles through bleak, uninhabited, craggy landscape, the Gruinard emerges by Gruinard House on Gruinard Bay, opposite Gruinard Island.
All but a short portion of the course of the river marks a boundary between Cromartyshire to the east and Ross-shire to the west. Much in the nature of Cromartyshire, it is not a boundary with a major body of the shire but with two small detached parts. At the rivermouth, the western shoreline is Ross-shire and the eastern shoreline Cromartyshire, as is the southernmost part of Gruinard Island opposite the rivermouth.
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