Columbia, river, c.1,210 mi (1,950 km) long, rising in Columbia Lake, SE British Columbia, Canada. It flows first NW in the Rocky Mt. Trench, then hooks sharply about the Selkirk Mts. to flow S through Upper Arrow Lake and Lower Arrow Lake and receive the Kootenai River (spelled Kootenay in Canada) before entering the United States after a course of 465 mi (748 km). It continues S through Washington and just below the mouth of the Spokane River is forced by lava beds to make a great bend westward before veering south again, running the while entrenched in a narrow valley through the Columbia Plateau. Its chief tributary, the Snake River, joins it just before it turns west again. The Columbia then forms part of the Washington-Oregon border before entering the Pacific Ocean through a wide estuary W of Portland, Oreg.
The Columbia River has created regal gorges by cutting through the Cascades and the Coast Ranges; it is fed by the Cowlitz and Willamette rivers, which drain the Puget trough between those ranges. Grand Coulee, now a reservoir in the Columbia basin project, was a former stream channel of the Columbia River. It was created during the last ice age when the Columbia's course was blocked by ice, forcing it to cut a new channel through the Columbia Plateau. When the ice receded the river resumed its former channel.
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