Back River, 974 km long, rises in Contwoyto Lake, north of Great Slave Lake, NWT, and flows northeast across the Barren Lands of Nunavut to Chantrey Inlet, south of King William Island. It has a drainage area of 106 500 km2 and a mean discharge of 612 m3/sec. The river is turbulent in its upper course, but as it enters a broad plain, it widens to form Lakes Pelly, Garry, Upper Macdougall and Lower Macdougall. On its final leg it narrows to cut through another rocky section to the coast. The Thelon Game Sanctuary lies along part of its length between the Consul and Baillie rivers.

The river is named for Sir George Back, who first explored it in 1834. The original name was Thlew-ee-choh, likely Dogrib for "great fish river." Back named the lakes after Sir John Henry Pelly and Nicholas Garry, governor and deputy governor of the Hudson's Bay Company, and his friend Lieutenant-Colonel Macdougall of the 79th Highlanders.