stylistic:tone
The Mackenzie River (French: Fleuve (de) Mackenzie; Slavey: Deh-Cho, literally big river; Inuvialuktun: Kuukpak, literally great river) is a river in the Canadian boreal forest. It forms, along with the Slave, Peace, and Finlay, the longest river system in Canada, and includes the second largest drainage basin of any North American river after the Mississippi.

The Mackenzie River flows through a vast, thinly populated region of forest and tundra entirely within the Northwest Territories in Canada, although its many tributaries reach into five other Canadian provinces and territories. The river's main stem is 1,738 kilometres (1,080 mi) long, flowing north-northwest from Great Slave Lake into the Arctic Ocean, where it forms a large delta at its mouth. Its extensive watershed drains about 20 percent of Canada. It is the largest river flowing into the Arctic from North America, and including its tributaries has a total length of 4,241 kilometres (2,635 mi), making it the thirteenth longest river system in the world.

The ultimate source of the Mackenzie River is Thutade Lake, in the Northern Interior of British Columbia. The Mackenzie valley is believed to have been the path taken by prehistoric peoples during the initial human migration from Asia to North America over 10,000 years ago, despite sparse evidence. The Inuvialuit, Gwich'in and other Indigenous peoples lived along the river for thousands of years. The river provided the major route into Canada's northern interior for early European explorers.

Economic development remains limited along the river. During the 19th century, fur trading became a lucrative business, but this was affected by harsh weather conditions. The discovery of oil at Norman Wells in the 1920s began a period of industrialization in the Mackenzie valley. Metallic minerals have been found along the eastern and southern edges of the basin; these include uranium, gold, lead, and zinc. Agriculture remains prevalent along the south, particularly in the Peace River area. Various tributaries and headwaters of the river have been developed for hydroelectricity production, flood control and irrigation.