Which river forms the border between the US states of Washington and Oregon?

The Columbia River forms most of the border between Washington and Oregon in the Pacific Northwest. After flowing south through Washington, it turns west and separates the two states before reaching the Pacific Ocean.

The river begins at Columbia Lake in British Columbia, Canada, and travels south into Washington. Near the Washington–Oregon boundary, it follows a broad westward course that defines the state line for much of its lower journey. Cities on or near this stretch include Portland, Oregon, and Vancouver, Washington.

The border is a result of 19th-century territorial history. The Oregon Treaty of 1846 divided the disputed Oregon Country along the 49th parallel, but the Columbia River became much of the boundary farther west. Oregon joined the Union in 1859, followed by Washington in 1889.

The Columbia is often confused with the Willamette, which flows through Oregon and joins the Columbia near Portland. The Snake River is its largest tributary, while the Fraser River is a separate major river in British Columbia. At its mouth, the Columbia enters the Pacific near Astoria, Oregon, and Long Beach, Washington.