What is the longest river in Canada, whose watershed is entirely within Canada?

The Mackenzie River is the longest river that's watershed is entirely within Canada. Arising from Thulade Lake in the Northwest Territories it flows northwest for 4,241 km to the Beaufort Sea.

The Mackenzie River flows through a vast boreal forest, in a thinly populated region of forest and tundra entirely within the Northwest Territories. The Mackenzie River drainage basin encompasses nearly 20 percent of Canada which is he second largest drainage basin of any North American river after the Mississippi.

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. The Inuvialuit, Gwich'in and other Indigenous peoples have 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 in the southern part of the watershed.