Winnipeg River, a river in Ontario and Manitoba, Canada. Winding through many lakes and rapids, the river flows for 200 miles (320 km) from the northern end of the Lake of the Woods to the southern end of Lake Winnipeg. The Winnipeg River's main tributary is the English River, which is the outlet of many large Ontario lakes. On the Winnipeg are several hydroelectric plants. The river was discovered about 1733 by Sieur de la Vérendrye and was the route of many later exploring expeditions.