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 in northern Ontario and tumbles over the Chaudière Falls at Ottawa where it 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.
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; fortunately, the residents had already been relocated due to the suspected instability of the earth in that location.
This river was an important trade route for the Algonquin people, who called it Kitchissippi, "Great River".
In 1615, Samuel de Champlain and Étienne Brűlé, 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.