River, eastern central Canada, the chief tributary of the St. Lawrence River .
It rises in the Laurentian plateau of western Quebec and flows west to form the Quebec-Ontario border before joining the St. Lawrence west of Montreal . It is 790 mi (1,271 km) long and forms innumerable lakes. Explored by Samuel de Champlain in 1613, it became a major route for explorers, fur traders, and missionaries to the Great Lakes . In the 19th century the Rideau Canal was completed, linking Ottawa to Lake Ontario , and lumbering became important. It is now a source of hydroelectric power.