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Watershed

Most of the Columbia's drainage basin (which, at 258,000 square miles or 670,000 square kilometres, is about the size of France) lies roughly between the Rocky Mountains on the east and the Cascade Mountains on the west. In the United States and Canada the term watershed is often used to mean drainage basin. The term Columbia Basin is used to refer not only to the entire drainage basin but also to subsets of the river's watershed, such as the relatively flat and unforested area in eastern Washington bounded by the Cascades, the Rocky Mountains, and the Blue Mountains. Within the watershed are diverse landforms including mountains, arid plateaus, river valleys, rolling uplands, and deep gorges. Grand Teton National Park lies in the watershed, as well as parts of Yellowstone National Park, Glacier National Park, Mount Rainier National Park, and North Cascades National Park. Canadian National Parks in the watershed include Kootenay National Park, Yoho National Park, Glacier National Park, and Mount Revelstoke National Park. Hells Canyon, the deepest gorge in North America, and the Columbia Gorge are in the watershed. Vegetation varies widely, ranging from western hemlock and western redcedar in the moist regions to sagebrush in the arid regions. The watershed provides habitat for 609 known fish and wildlife species, including the bull trout, bald eagle, gray wolf, grizzly bear, and Canada lynx.

The World Wide Fund for Nature (WWF) divides the waters of the Columbia and its tributaries into three freshwater ecoregions: Columbia Glaciated, Columbia Unglaciated, and Upper Snake. The Columbia Glaciated ecoregion, about a third of the total watershed, lies in the north and was covered with ice sheets during the Pleistocene. The ecoregion includes the mainstem Columbia north of the Snake River and tributaries such as the Yakima, Okanagan, Pend Oreille, Clark Fork, and Kootenay rivers. The effects of glaciation include a number of large lakes and a relatively low diversity of freshwater fish. The Upper Snake ecoregion is defined as the Snake River watershed above Shoshone Falls, which totally blocks fish migration. This region has 14 species of fish, many of which are endemic. The Columbia Unglaciated ecoregion makes up the rest of the watershed. It includes the mainstem Columbia below the Snake River and tributaries such as the Salmon, John Day, Deschutes, and lower Snake Rivers. Of the three ecoregions it is the richest in terms of freshwater species diversity. There are 35 species of fish, of which four are endemic. There are also high levels of mollusk endemism.

In 2016, over eight million people lived within the Columbia's drainage basin. Of this total about 3.5 million people lived in Oregon, 2.1 million in Washington, 1.7 million in Idaho, half a million in British Columbia, and 0.4 million in Montana. Population in the watershed has been rising for many decades and is projected to rise to about 10 million by 2030. The highest population densities are found west of the Cascade Mountains along the I-5 corridor, especially in the Portland-Vancouver urban area. High densities are also found around Spokane, Washington, and Boise, Idaho. Although much of the watershed is rural and sparsely populated, areas with recreational and scenic values are growing rapidly. The central Oregon county of Deschutes is the fastest-growing in the state. Populations have also been growing just east of the Cascades in central Washington around the city of Yakima and the Tri-Cities area. Projections for the coming decades assume growth throughout the watershed. The Canadian part of the Okanagan subbasin is also growing rapidly.

Climate varies greatly within the watershed. Elevation ranges from sea level at the river mouth to more than 14,000 feet (4,300 m) in the mountains, and temperatures vary with elevation. The highest peak is Mount Rainier, at 14,411 feet (4,392 m). High elevations have cold winters and short cool summers; interior regions are subject to great temperature variability and severe droughts. Over some of the watershed, especially west of the Cascade Mountains, precipitation maximums occur in winter, when Pacific storms come ashore. Atmospheric conditions block the flow of moisture in summer, which is generally dry except for occasional thunderstorms in the interior. In some of the eastern parts of the watershed, especially shrub-steppe regions with Continental climate patterns, precipitation maximums occur in early summer. Annual precipitation varies from more than 100 inches (250 cm) a year in the Cascades to less than 8 inches (20 cm) in the interior. Much of the watershed gets less than 12 inches (30 cm) a year.

Several major North American drainage basins and many minor ones border the Columbia River's drainage basin. To the east, in northern Wyoming and Montana, the Continental Divide separates the Columbia watershed from the Mississippi-Missouri watershed, which empties into the Gulf of Mexico. To the northeast, mostly along the southern border between British Columbia and Alberta, the Continental Divide separates the Columbia watershed from the Nelson - Lake Winnipeg - Saskatchewan watershed, which empties into Hudson Bay. The Mississippi and Nelson watersheds are separated by the Laurentian Divide, which meets the Continental Divide at Triple Divide Peak near the headwaters of the Columbia's Flathead River tributary. This point marks the meeting of three of North America's main drainage patterns, to the Pacific Ocean, to Hudson Bay, and to the Atlantic Ocean via the Gulf of Mexico.

Further north along the Continental Divide, a short portion of the combined Continental and Laurentian divides separate the Columbia watershed from the Mackenzie - Slave - Athabasca watershed, which empties into the Arctic Ocean. The Nelson and Mackenzie watersheds are separated by a divide between streams flowing to the Arctic Ocean and those of the Hudson Bay watershed. This divide meets the Continental Divide at Snow Dome (also known as Dome), near the northernmost bend of the Columbia River.

To the southeast, in western Wyoming, another divide separates the Columbia watershed from the Colorado – Green watershed, which empties into the Gulf of California. The Columbia, Colorado, and Mississippi watersheds meet at Three Waters Mountain in the Wind River Range of Wyoming. To the south, in Oregon, Nevada, Utah, Idaho, and Wyoming, the Columbia watershed is divided from the Great Basin, whose several watersheds are endorheic, not emptying into any ocean but rather drying up or sinking into sumps. Great Basin watersheds that share a border with the Columbia watershed include Harney Basin, Humboldt River, and Great Salt Lake. The associated triple divide points are Commissary Ridge North, Wyoming, and Sproats Meadow Northwest, Oregon. To the north, mostly in British Columbia, the Columbia watershed borders the Fraser River watershed. To the west and southwest the Columbia watershed borders a number of smaller watersheds that drain to the Pacific Ocean, such as the Klamath River in Oregon and California and the Puget Sound Basin in Washington.