The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada. For more than ten millennia, the Columbia River has been the most important and intensively used part of Oregon’s natural landscape. The river’s main stem gathers water from ten principal tributaries that drain 259,000 square miles in seven U.S. states and one Canadian province.
In Oregon, the largest tributaries are the Snake, Willamette, Deschutes, John Day, and Umatilla Rivers. Each arises in mountainous terrain and carries significant minerals in suspension to create alluvial valleys, notably the Willamette, one of the most fertile regions in western North America.
Because high mountain glaciers and annual snowpack feed the Columbia, the main stem flushes an enormous volume of water, especially during the spring thaw, making it the fourth largest in flow among North American rivers. Its annual average discharge is 198 million acre-feet of water, with the highest volumes occurring between April and September, the lowest from December to March.
The Columbia main stem is 1,249 miles long, originating in Columbia Lake in southeastern British Columbia at 2,656 feet above sea level. The river flows north on the east side of the Selkirk Mountains for 200 miles before turning south and running through a series of gorges to the international border. Coursing southwest through north-central Washington State, the river skirts massive lava flows on the Columbia Plateau before turning southeast and then west to the Pacific. Fifteen percent of the basin—39,000 square miles—is in Canada.
The Columbia’s largest tributary, the Snake River, flows more than a thousand miles from Two-Oceans Plateau in Yellowstone National Park, across the Snake River Plain and into the deepest canyon in North America, Hells Canyon, on the eastern boundary of Oregon. The Snake joins the Columbia in southeastern Washington, just miles from Oregon’s northern boundary.The Columbia’s main stem drops an average of more than two feet per mile, but in the middle section of basalt-lined gorges it drops more than five feet per mile. The most spectacular section is a 100-mile-long gorge, some 50 miles east of Portland. The Columbia River Gorge cuts through the Cascade Mountain Range, making the Columbia the only Pacific tributary to the Pacific to cut through western mountains at near sea level. More than 100 massive floods carved out the 3,000-foot trough between 15,000 and 18,000 years ago. The Gorge is also the site of one of the planet’s sharpest climatological divides, where precipitation varies from 75 to 15 inches per year in less than 30 miles.
The reach of tide on the Columbia River extends from the Pacific a little more than 100 miles from the ocean to the Gorge. This flat lower river section, which falls less than one-half foot per mile, includes Sauvie Island, one of the largest river islands in North America.