The Columbia River forms a significant portion of the border between Washington and Oregon.

The Columbia River, the largest river in the Pacific Northwest, serves as the natural boundary between Washington and Oregon for much of its length. Originating in the Canadian Rockies, it flows through Washington before turning south to delineate the border with Oregon. This river is essential for transportation, irrigation, and hydroelectric power, with dams like the Bonneville Dam providing significant energy resources. The Columbia also supports diverse ecosystems and is vital to the region's salmon population, playing a key role in the culture and economy of both states. Its scenic beauty makes it a popular destination for recreation and tourism.