The Ottawa River watershed has a total area of 148 000 km2. From forested, rocky uplands of the Canadian Shield in Quebec, the river flows westward to Lake Timiskaming, then southeastward along the Ontario-Quebec border, through the agricultural lands of the lower Ottawa valley, and finally joins the St. Lawrence River. The river traverses a total distance of over 1130 km and descends about 400 m, from an elevation of 430 m at the headwaters to 20 m at its mouth. Flow is managed through regulation of its principal reservoirs by a joint federal-provincial planning board.
Rapids and waterfalls, beaches and islands!
Falls and rapids occur where the Ottawa River drops over resistant bedrock outcrops. At these locations the river is relatively shallow and swift, and bedrock islands may divide the channel. Hydroelectric dams exploit some of these natural drops near Portage-du-Fort, Fitzroy Harbour, and Chaudière Falls, as well as along many of the tributary rivers (e.g. Gatineau River). Natural and artificial beaches are present in places along the wider, slower flowing reaches of the Ottawa River (Norway beach, Britannia beach). Downstream from the confluence with the Gatineau River, the Ottawa River flows across Quaternary sediments and the landscape changes. Here, vegetated sandbars form low islands (Kettle Island, Petrie Island) and marshes are common along the shore.
A precious resource
The Ottawa River is the source of drinking water for many local communities. The City of Ottawa is the heaviest of these users, drawing 341 million litres of water from the river daily at the Britannia and Lemieux Island purification plants. Historically, the Ottawa River has been a transportation route for native peoples, fur traders, and the timber industry. In the 1840s, saw and grist mills were located at Chaudière Falls. This site later developed into a sprawling complex of hydroelectric plants and saw, pulp and paper, and carbide mills, all utilizing the water and exploiting the energy of the falls. Some of these industries remain in operation today. Recreation is now a major use of the river.
The runoff cycle
Flow varies considerably throughout the year. High runoff during the spring is caused by snowmelt. This is followed by lower flow during the drier summer months, an increase in flow due to fall rains, and fairly steady flow through the winter.
The lower Ottawa River experiences two spring flood peaks. Spring arrives earlier along the southern tributaries (Mississippi, Rideau, and South Nation rivers) which causes the Ottawa River to rise to its first peak. The second, and normally higher, peak arrives about three weeks later from snowmelt in the northern part of the basin.