The river rises from its source in Lake Capimitchigama, in the Laurentian Mountains of central Quebec, and flows west to Lake Timiskaming. From there its route has been used to define the interprovincial border with Ontario.
From Lake Timiskaming, the river flows southeast to Ottawa and Gatineau, where it tumbles over the Chaudière Falls and further takes in the Rideau and Gatineau rivers.
The Ottawa River drains into the Lake of Two Mountains and the St. Lawrence River at Montreal. The river is 1,271 kilometers (790 mi) in length; it drains an area of 146,300 km2, 65% in Quebec and the rest in Ontario, with a mean discharge of 1,950 m3/s.
The average annual mean waterflow measured at Carillon dam, near the Lake of Two Mountains, is 1,939 m3/s, with average annual extremes of 749 to 5,351 m3/s. Record historic levels since 1964 are a low of 529 in 2005 and a high of 8190 m3/s in 1976.
The river flows through large areas of deciduous and coniferous forest formed over thousands of years as trees recolonized the Ottawa Valley after the ice age. Generally, the coniferous forests occur on old sand plains left by retreating glaciers, or in wetter areas with clay substrate. The deciduous forests, dominated by maple, beech, oak and ash occur in more mesic areas with better soil. These primeval forests were occasionally affected by natural fire, mostly started by lightning, which led to increased reproduction by pine and oak, as well as fire barrens and their associated species. The vast areas of pine were exploited by early loggers. Later generations of logging removed hemlock for use in tanning leather, leaving a permanent deficit of hemlock in most forests. Associated with the logging and early settlement were vast wild fires which not only removed the forests, but led to soil erosion. Consequently nearly all the forests show varying degrees of human disturbance. Tracts of older forest are uncommon, and hence they are considered of considerable importance for conservation.
The Ottawa River has large areas of wetlands. Some of the more biologically important wetland areas include (going downsteam from Pembroke), the Westmeath sand dune/wetland complex, Mississippi Snye, Breckenridge Nature Reserve, Shirleys Bay, Ottawa Beach/Andrew Haydon Park, Petrie Island, the Duck Islands and Greens Creek. The Westmeath sand dune/wetland complex is significant for its relatively pristine sand dunes, few of which remain along the Ottawa River, and the many associated rare plants. Shirleys Bay has a biologically diverse shoreline alvar, as well as one of the largest silver maple swamps along the river. Like all wetlands, these depend upon the seasonal fluctuations in the water level. High water levels help create and maintain silver maple swamps, while low water periods allow many rare wetland plants to grow on the emerged sand and clay flats. There are five principle wetland vegetation types. One is swamp, mostly silver maple. There are four herbaceous vegetation types, named for the dominant plant species in them: Scirpus, Eleocharis, Sparganium and Typha. Which type occurs in a particular location depends upon factors such as substrate type, water depth, ice-scour and ferility. Inland, and mostly south of the river, older river channels, which date back to the end of the ice age, and no longer have flowing water, have sometimes filled with a different wetland type, peat bog. Examples include Mer Bleue and Alfred Bog.
Major tributaries include:
- Bonnechere River
- Coulonge River
- Dumoine River
- Gatineau River
- Kipawa River
- du Lièvre River
- Madawaska River
- Mattawa River
- Mississippi River
- Montreal River
- Rivière du Nord
- Noire River
- Petawawa River
- Rideau River
- Rouge River
- South Nation River
communities along the Ottawa River include (in down-stream order):
Read more about this topic: Ottawa River
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