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The Ottawa River (rivière des Outaouais) is a Canadian river which for most of its length defines the border between the provinces of Ontario and Quebec. It rises from its source in Lake Capimitchigama in western Quebec, reaches the Ontario border at Lake Timiskaming, flowing southeast to Ottawa, where it tumbles over the Chaudière Falls and further takes in the Rideau River and Gatineau River. Between the Chaudière Falls and Rideau Falls lies Parliament Hill perched atop a steep and militarily strategic gorge. The Ottawa River drains into the Saint Lawrence River at Montreal, forming Lac Saint-Louis. The length of the river is 1,271 km and it drains an area of 146,000 km².

 Following the retreat of the glaciers from this area at the end of the last ice age, the Ottawa River valley was flooded by an arm of the Atlantic Ocean known as the Champlain Sea. Fossil remains of marine life have been found in marine clay formed during that time. Sediment deposits from this period have resulted in areas of poor drainage and the presence of large bogs in some ancient channels of this river. Another consequence was the formations of large deposits of a material commonly known as "Leda clay"; these deposits become highly unstable after heavy rains. A number of landslides have occurred as a result. The former site of the town of Lemieux, Ontario collapsed into the South Nation River in 1993; however, the residents had already been relocated due to the suspected instability of the earth in that location.
Large numbers of Canada Geese, ducks, gulls and shorebirds take advantage of spring flooding and wetlands in the Ottawa River valley during migration. Ducks also overwinter in sections of the river that do not freeze over.

•    History

This river was an important trade route for the Algonquin people, who called it Kitchissippi, "Great River". Some early European explorers, possibly considering the Ottawa River to be more significant than the Upper Saint Lawrence, applied the name River Canada to the Ottawa River and the Saint Lawrence River below the confluence at Montreal. As the extent of the Great Lakes became clear and the river began to be regarded as a tributary, it was variously known as the Grand River, "Great River" or Grand River of the Algonquins before the present name was settled upon. This name change resulted from Ottawa peoples' control of the river circa 1685. However, only one band of Ottawa, the Kinouncherpirini or Keinouch, ever inhabitted the Ottawa Valley.
In 1615, Samuel de Champlain and Étienne Brûlé, assisted by Algonquin guides, were the first Europeans to travel up the Ottawa River and follow the water route west to Georgian Bay that would be used by French fur traders for the next two centuries.
In the early 19th century, the Ottawa River and its tributaries were used to gain access to large untouched forests of white pine. A booming trade in timber developed and large rafts of logs were floated down the river. In 1832, the Ottawa River was connected to Lake Ontario via the construction of the Rideau Canal.
A number of hydroelectric dams have been constructed on this river. However, it is still extensively used for recreational boating.