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 and the MacKenzie River, 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 RockyMountain Range in Canada. Its largest tributary, the Snake, travels from its source in Yellowstone National Park in Wyoming before joining the Columbia. When Lewis and Clark explored the region in the early 19th century, huge numbers of fish returned to spawn every year. "The multitudes of this fish are almost inconceivable," Clark wrote in the autumn of 1805. At that time, the Columbia and its tributaries provided of pristine river habitat.
Common usage
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. At its center is the Pasco Basin, an area roughly double the size of, and fully containing, the Hanford Site.
