Which river separates the US states of Oregon and Washington?

The Columbia River separates the US states of Oregon and Washington. It flows south through Washington before turning west and forming much of the boundary between the two states on its way to the Pacific Ocean.

The river is about 1,243 miles long and begins in the Rocky Mountains of British Columbia, Canada. Its watershed covers parts of seven US states and one Canadian province, making it one of the Pacific Northwest’s defining geographic features. The Columbia River Gorge is a particularly dramatic section of the border.

A common mix-up is the Willamette River, which flows through Oregon and joins the Columbia near Portland but does not separate Oregon from Washington. The Snake River is the Columbia’s largest tributary, while the Fraser River lies primarily in British Columbia.

The Columbia also shaped regional history. The 1846 Oregon Treaty established the 49th parallel farther east as part of the US–British boundary, while the Columbia itself became much of the later state line between Oregon and Washington.