The Columbia Basin includes the southeastern portion of the
Canadian province of
British Columbia, most of the
U.S. states of
Idaho,
Oregon, and
Washington, the western part of
Montana, and very small portions of
Nevada,
Utah, and
Wyoming. The south and southeastern drainage
divide borders the interior drainage of the northern
Great Basin. To the northeast the region borders the basins of the
Saskatchewan River (
Hudson Bay) and the
Mackenzie River (
Beaufort Sea), and to the northwest the basin of the
Fraser River. The Columbia Basin extends from the
Rocky Mountains in the east through the
Cascade Range to the Columbia River's outflow at the
Pacific Ocean in the west. The Columbia River pours more water into the Pacific Ocean than any other river in North or South America. In its course to the Pacific Ocean, the Columbia flows through four mountain ranges—the
Rockies,
Selkirks,
Cascades, and coastal mountains—and drains . The mainstem of the Columbia rises in
Columbia Lake on the west slope of the Rocky Mountain Range in Canada. Its largest tributary, the
Snake, travels from its source in
Yellowstone National Park in Wyoming before joining the Columbia.
Washington region Residents of the area surrounding the confluence of the
Columbia and
Snake rivers—a region centering on the
Tri-Cities, Washington metropolitan area—use the term "Columbia Basin" to refer to their own, much smaller region. This usage is roughly synonymous with the
Columbia Plateau or roughly equivalent to the relatively unforested area bounded by the
Cascades,
Blue,
Wallowa, and
Rocky mountain ranges and the
Okanagan Highland. This sense of the term Columbia Basin has expanded from its early focus on the land irrigated by
Grand Coulee Dam and the
Columbia Basin Project to include other irrigation districts such as the
Yakima and
Walla Walla valleys. The area includes valuable farmland that has excellent soil profile and underlying silty loess. At its center is the
Pasco Basin, an area roughly double the size of, and fully containing, the
Hanford Site.
Oregon region Many major rivers that flow through Oregon are tributaries of the Columbia River, including the Umatilla River and John Day River in northeastern Oregon. The largest tributaries from the south are the
Deschutes River in
Central Oregon, which provides much of the drainage on the eastern side of the
Cascade Range, and the
Willamette River, which flows between the
Oregon Coast Range and the
Cascade Range. The Willamette accounts for 12 to 15 percent of the Columbia's flow, and the basin that contains two-thirds of Oregon's population, including the state capital,
Salem, Oregon, and the state's largest city,
Portland, Oregon which surrounds the Willamette's mouth at the Columbia. ==History==