Ottawa River, river in east-central Canada, the chief tributary of the St. Lawrence River. It rises in the Laurentian Plateau of western Quebec and flows swiftly westward to Lake Timiskaming and then southeastward, forming for most of its course the Quebec–Ontario provincial border before it joins the St. Lawrence west of Montreal. Through its total course of 790 mi (1,271 km), the river forms innumerable lakes, the largest being Grand Victoria, Simard, Timiskaming, Allumette, Chats, and Deschenes. The Ottawa and its main tributaries, including the Rouge (115 mi long), Lièvre (205), Gatineau (240), Coulonge (135), Rideau (91), Mississippi (105), and Madawaska (143) rivers, drain an area of more than 55,000 sq mi (142,000 sq km).

Explored in 1613 by Samuel de Champlain and named for a band of Algonkin Algonquin Indians that once inhabited the area, the river became a chief route of explorers, fur traders, and missionaries to the Upper Great Lakes. In the 19th century the Lumbering became the dominant activity along the river in the early 19th century, and by mid-century it was the economic engine of the region. In 1832 the Rideau Canal, linking Ottawa to Lake Ontario, was completed, and lumbering became the dominant activity along the river. The river is no longer a major transportation artery, but it is an important source of hydroelectric power; several hydro plants and an atomic energy plant at Chalk River supply electricity for much of Quebec and Ontario. Riverine cities include Pembroke and Ottawa in Ontario and Hull in Quebec.