The Columbia River, 2000 km long (of which 801 km are in Canada), rises in Columbia Lake (elevation 820 m) in southeast British Columbia. It then flows north-northwest in a sharp detour around the Selkirk range before abruptly turning south past Revelstoke, through Upper and Lower Arrow Lakes, past Castlegar, and across the US border (at elevation 390 m). This route was followed by the Big Bend Highway, which opened in 1940 and provided the shortest route through the Rockies until the Rogers Pass section was opened. Just north of the border the river is joined by the Kootenay River, which also rises in the area of Columbia Lake, loops south into the US and north again into Kootenay Lake, before heading west to join the Columbia.

The Columbia continues south across Washington state, but is forced off its natural course by a lava flow and glacial debris into a second huge curve, also called the "Big Bend." At the top of the bend, it is joined by the Okanagan River (314 km), which drains the lakes of the Okanagan Valley. Just north of the Oregon border it is joined by the Snake River and makes a right-angled turn, flowing west to the Pacific below Portland, Oregon. The Columbia is a long, powerful river which has cut deep gorges along much of its course, and it commands one of the greatest drainage basins in North America, totalling 155 000 km2 (including 51 800 km2 in the US). The average flow at the international boundary is 2800 m3/s.

Originally called Rio de San Roque by Spanish explorers, it was rediscovered by the Boston trader Captain Robert Gray, who named it after his ship, the Columbia. (The river may be unique in being named after a ship.) David Thompson of the North West Company was the first to explore it from its headwaters to its mouth (1811), but American traders had preceded him overland, and were already at work on Fort Astoria. The Hudson's Bay Company later built Fort Vancouver (1824-25) 150 kilometres inland from its mouth, and as the only water route to the interior from the Pacific, the river served as the major transportation artery until the railways came. The Columbia became the de facto boundary between British and American territory, but by the 1840s American immigration to the area was ascendant, and the British agreed to set the border at the 49th parallel.

Though its headwaters are in Canada, most of the river's development has been in the US. By the Columbia River Treaty (1961; ratified by Canada 1964), Canada agreed to build 3 storage dams: the Duncan (1967) north of Kootenay Lake, Hugh Keenleyside (1968) on the Columbia, and Mica (1973) north of Revelstoke. The Mica project, which began generating 1976-77, has a nameplate capacity of 1736 MW, and the total potential of the river is estimated to be 4000 MW in Canada.

The agreement has been controversial because of the environmental disruption of the dams and because the irrigation and hydroelectric power mostly benefit the US. However, Canada shares the power and revenue. Some of the world's largest hydroelectric generators are on the river; eg, at Grand Coulee. The dams have greatly impeded the salmon runs. The Columbia was one of the world's great spawning grounds.