List of longest rivers of Canada
Nine rivers in this list cross international boundaries or form them. Four—the Yukon, Columbia, Porcupine, and Kootenay—begin in Canada and flow into the United States. Five—the Milk, Pend d'Oreille, Saint Lawrence, Red, and Saint John—begin in the United States and flow into Canada. Of these, the Milk and the Kootenay cross the international border twice, the Milk leaving and then re-entering the United States, the Kootenay leaving and then re-entering Canada. The drainage basins of these nine rivers extend into both countries; in addition, the drainage basins of six others—the Fraser, Assiniboine, South Saskatchewan, Saskatchewan, Nelson, and Winnipeg—extend into the United States even though their main stems flow entirely within Canada.
Mackenzie River
The Mackenzie drainage basin is bordered by multiple major North American watersheds. Much of the western edge of the Mackenzie basin runs along the Continental Divide. The divide separates the Mackenzie watershed from that of the Yukon River, which flows to the Bering Strait; and the Fraser River and Columbia River systems, both of which empty into the Pacific Ocean. Lowland divides in the north separate the Mackenzie basin from the Arctic Ocean watersheds of the Anderson, Horton, Coppermine and Back Rivers. In the east, the Mackenzie borders on the Hudson Bay watersheds of the Thelon and Churchill Rivers, and to the south it is bordered by the Saskatchewan–Nelson River system, which also empties into Hudson Bay. The Mackenzie system is hydrologically connected to the Hudson Bay watershed via Wollaston Lake, which is not only the source of the Fond du Lac tributary of Lake Athabasca, but also of the Cochrane River, which flows east into the Churchill River.