Ottawa, river, c.700 mi (1,130 km) long, largest tributary of the St. Lawrence River, Canada. It rises in the Laurentian Highlands, SW Que., and flows generally W through La Vérendrye Provincial Park to Lake Timiskaming, then SE forming part of the Quebec-Ontario border, past Ottawa, and into the St. Lawrence River near Montreal. Its lower course has several expansions, known as the Allumetter, Chats, and Deschênes lakes and Lake of the Two Mountains. Among its chief tributaries are the Gatineau, Lièvre and Coulonge rivers. Hydroelectric power stations at La Cave, Des Joachims, Bryson, Chenaux, Chats, Chaudière Falls, and Carillion have a combined generating capacity of about 1.5 million kW. The river is navigable for large vessels as far as Ottawa; it is connected with Lake Ontario by the Rideau Canal system. There is some farming in the valley below Pembroke, but lumbering is the chief industry along the lower river. Samuel de Champlain, the French explorer, was the first European to visit (1613–15) the valley; the river, known then as the Grand River, later became an important highway for fur traders and missionaries.
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