Mackenzie, river in the Inuvik and Fort Smith regions of the Northwest Territories, northwestern Canada, 1,800 km (1,120 mi) long. The Mackenzie rises in Great Slave Lake and flows northwest before emptying through a large delta into Mackenzie Bay, an arm of the Beaufort Sea. It is navigable from June to October, when it freezes over. The Mackenzie Valley, between the Mackenzie Mountains and the Canadian Shield, is sparsely populated and has few economic resources, although petroleum is produced near Norman Wells and oil and natural gas have been found in the delta region. Muskrat-trapping is also important in the delta region. The Mackenzie is part of the longest river system in Canada, 4241 km (2635 mi) long. The river was explored in 1789 by Sir Alexander Mackenzie, for whom it is named. Steamers operated on the river from the 1880s until the 1940s.