The Ottawa River is a river in the Canadian provinces of Ontario and Quebec. For most of its length, it defines the border between these two provinces.
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 kilometres long; it drains an area of 146,300 square kilometres, 65 percent in Quebec and the rest in Ontario, with a mean discharge of 1,950 cubic metres per second.