river in northwestern Canada. It rises in the Saint Cyr Range of the Pelly Mountains, Yukon Territory, and flows southeast into British Columbia, then northeast to join the Mackenzie River at Fort Simpson in the Northwest Territories, after a course of 693 miles (1,115 km). Its upper course is characterized by rapids and canyons; its lower course is navigable for small boats from Fort Simpson to Fort Liard, 165 miles upstream. Part of the river's valley is followed by the Alaska Highway. Its tributaries include the Hyland, Kechika, Coal, Beaver, Petitot, Fort Nelson, and South Nahanni rivers. It is named for the liards (poplar trees) along its course.