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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.

Major tributaries include the:

• Bonnechere RiverThe Bonnechere River is a river in eastern Ontario, Canada. It starts in Algonquin Park and flows east through Renfrew County, emptying into the Ottawa River east and north of Renfrew, Ontario. The river is 145 km in length and drains an area of 2,400 km²
• Coulonge River
• Gatineau River
• Kipawa River
• du Lièvre River
• Madawaska RiverThis article is about the Madawaska River in Ontario. For other rivers named Madawaska, see Madawaska River (disambiguation). Arnprior. The Madawaska River is a river in Ontario, Canada. The river is 230 km long and drains an area of 8,470 km². It rises i
• Mississippi RiverThe Mississippi River is a tributary of the Ottawa River, approximately 160 km (100 mi) long, in southern Ontario in Canada. It rises east of the Kawartha Lakes and flows northeast through Dalhousie Lake and Mississippi Lake, past Carleton Place, through
• Montreal RiverThere are two rivers named Montreal River in Ontario, Canada: The Montreal River in eastern Ontario is a tributary of the Ottawa River, emptying into Lake Timiskaming. The Lady Evelyn River is a tributary of this river. The Montreal River in the Algoma Di
• Petawawa RiverThe Petawawa River is a river in eastern Ontario, Canada. It starts in northern Algonquin Park and flows east, emptying into the Ottawa River at Petawawa. The river is 187 km in length and drains an area of 4,200 km². The river's name comes from the Algon
• Rideau River
• Rouge River
• South Nation RiverThe South Nation River is a river whose watershed covers 3,900 km², a large part of eastern Ontario. The river's headwaters are located north of Brockville, Ontario and it flows 175 km northeast to empty into the Ottawa River near Plantagenet, Ontario.

Following the retreat of the glacierA glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity. Equivalently, it is a multi-year ice accretion in mountainous terrain. The glacier fringe is the area where the glacier has recently melted. There ares 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.