A significant river of the N Highlands, the River Cassley emerges from Loch na Sroine Luime, having formed within a group of lochs thereabouts; namely Fionn Loch Mor, Fionn Loch Beag and Gorm Loch Mor. The Cassley flows south southeast, gaining water from numerous tributary streams, the largest of which are the Allt a' Chnaip Ghiubhais, Allt Loch Carn nan Conbhairean, Abhainn Gleann na Muic, Allt Langwell and Allt an Rasail, before falling into the River Oykel at Invercassley, having completed a course of 15 miles (24 km).
The upper reaches of the river are modified to provide water for the Duchally hydro-electric power station, located 2½ miles (4 km) north northwest of Duchally Lodge, which is part of the Shin Hydro-Electric Power Scheme. The Achness Waterfall lies on the lower reaches of the river.
