What is Canada's longest river?

The Mackenzie River is Canada's longest river, flowing about 1,738 kilometres from Great Slave Lake in the Northwest Territories north to the Beaufort Sea of the Arctic Ocean. Including its principal source streams (the Peace River and the Finlay River, which rise in the British Columbia Rockies), the entire Mackenzie River system is the second-longest in North America at about 4,241 kilometres, behind only the Mississippi-Missouri system. The river drains about 1.8 million square kilometres, including most of the Northwest Territories and parts of British Columbia, Alberta, Saskatchewan, and Yukon.

The river is named after Sir Alexander Mackenzie, a Scottish-born North West Company fur trader who reached the river's mouth on July 14, 1789 in search of a Pacific passage. Mackenzie was disappointed to discover that the river flowed to the Arctic rather than the Pacific (he reportedly called it the River of Disappointment), but his expedition was the first European transcontinental crossing of North America. The Dene peoples have lived along the river for thousands of years and call it Deh-Cho (Big River); the Inuvialuit at the mouth of the river call it Kuukpak.

The Mackenzie River system passes through major settlements at Fort Providence, Wrigley, Tulita, Norman Wells (the historic site of the Canol Project oil pipeline of 1942 to 1944), Fort Good Hope, Tsiigehtchic, Inuvik, and Aklavik. The Mackenzie Delta at the river's mouth covers about 12,000 square kilometres and is one of the largest deltas in the world. The delta is a major waterfowl breeding area, with millions of geese, ducks, and shorebirds using the area annually. Polar bears, beluga whales, ringed seals, and bowhead whales are common in the Beaufort Sea offshore.

The river is navigable by tugs and barges from Hay River south of Great Slave Lake to Inuvik between June and October, providing the Mackenzie Highway shipping route that supplies many remote communities. The Inuvik-Tuktoyaktuk Highway, opened on November 15, 2017, is the only road that reaches the Arctic Ocean within Canada and crosses the Mackenzie Delta. The proposed Mackenzie Valley Pipeline, repeatedly delayed since the 1970s and the subject of the Berger Inquiry of 1974 to 1977, would have transported natural gas from the Beaufort Sea to southern markets but has not been built. Climate change is affecting the Mackenzie system, with permafrost thaw, earlier ice break-up, and altered fish populations all documented since the 1990s.