Mackenzie River, a river of British North America, in the Northwest territories of Canada, which has its head in Great Slave lake, and, after a N. course of about 1,200 m., empties through several mouths into the Arctic ocean, in about lat. 69° N., Ion. 135° W. It flows through beds of coal and lignite, and is navigable by steamers throughout, though the rapids below Great Bear river in times of low water might prove an obstacle. Its width, which is irregular, sometimes extends to two miles. The ice breaks up at Fort Simpson, one of the Hudson Bay company's posts, in lat. 61° 51', about the beginning or middle of May, and the river is open to its mouth about the end of that month. Floating ice generally prevents navigation even in the upper part of its course before the beginning of June. The chief tributaries of the Mackenzie are Mountain river or Riviere aux Liards from the west, and Great Bear river, the outlet of Great Bear lake, from the east. Mountain river rises beyond the Rocky mountains near the sources of the Yukon, and empties into the Mackenzie at Fort Simpson; it has a rapid current, and its navigation is dangerous. - The Mackenzie is but a part of a stream 2,500 m. in length, the upper portions of which bear different names.

It rises as the Athabasca in the Rocky mountains, near the source of the Columbia, and, after an extremely rapid descent through a fertile and well wooded country, receiving the waters of Lesser Slave lake from the west, and a little further down successively those of Lac La Biche and the Clearwater or Little Athabasca river from the east, enters the S. W. extremity of Lake Athabasca. The portion between the affluent from Lac La Biche and the Clearwater river is called riviere a la Biche or Red Deer river. After leaving Lake Athabasca it is called first the Rock and then the Slave river, until it loses itself in Great Slave lake. The Hudson Bay company's boats ascend with only two interruptions to Jasper House, more than 2,000 m. from the Arctic ocean. These interruptions are a group of rapids in riviere a la Biche, and another in Slave river. Just below Lake Athabasca the Peace river enters from the west. This stream, which is considered by some the source of the Mackenzie, rises in the Rocky mountains near the source of Fraser river, and flows through a beautiful and fertile valley; it is navigable by the Hudson Bay company's boats throughout nearly its entire course. - The Mackenzie river was discovered and first navigated in 1789 by Alexander Mackenzie, from whom it derives its name.