What is the name of the vital waterway of the Pacific Northwest that forms much of the Washington-Oregon border?

The vital waterway of the Pacific Northwest that forms much of the Washington-Oregon border is the Columbia River. This river is significant not only for its geographical role in delineating state boundaries but also for its historical importance in the exploration and development of the region. Spanning over 1,200 miles, the Columbia River begins in the Rocky Mountains of British Columbia and flows through Washington and Oregon before emptying into the Pacific Ocean.

Its basin is crucial for transportation, hydroelectric power generation, and irrigation, making it a key resource for the surrounding communities. The river has played an essential role in the local economy and has been a site for cultural significance for various Indigenous tribes for thousands of years.

In contrast, the other rivers listed such as the Missouri, Snake, and Red do not form the boundary between Washington and Oregon, nor do they hold the same historical and economic importance within the context of the Pacific Northwest. The Missouri River is primarily located in the central United States, the Snake River flows primarily through Idaho and parts of Wyoming, and the Red River mainly flows along the Texas-Oklahoma border. Thus, the Columbia River is accurately recognized as the central waterway forming much of the border between Washington and Oregon.