Winnipeg River, 813 km long (to head of Firesteel River), issues from the north end of Lake of the Woods and flows northwest to Lake Winnipeg. The river takes its name from the lake, which the Cree called win-nipi, meaning "murky water." After its discovery by Jean-Baptiste de La Vérendrye (1733) the river was a major link in the fur trading route between Lakes Superior and Winnipeg. Both the North West Company and Hudson's Bay Company built forts on the river, but traffic dwindled after the merger of the 2 companies in 1821.

The Winnipeg River has a heavy, rapid flow and was first used for power by a lumber mill at Fort Alexander (now Pine Falls) near the mouth of the river in 1870. Its first hydroelectric power was fed to pulp mills at Kenora, Ontario, in 1892, and the first hydroelectric plant on the Manitoba reach began construction at Pinawa in 1903. Today, 7 (6 in Manitoba and one in Ontario) generating stations harness all but a few metres of the river's 106 m drop. The once turbulent river (explorer Alexander Mackenzie called it the White River because of its many rapids), with its 30 or more portages, is now placid, though still frequented by boats and canoes. From 1965 to 1985 its water was used to cool organic coolant in the Whiteshell Nuclear Research Establishment (now Whiteshell Laboratories).