<P> The Columbia River is the largest river in the Pacific Northwest region of North America . The river rises in the Rocky Mountains of British Columbia , Canada . It flows northwest and then south into the US state of Washington , then turns west to form most of the border between Washington and the state of Oregon before emptying into the Pacific Ocean . The river is 1,243 miles ( 2,000 km ) long , and its largest tributary is the Snake River . Its drainage basin is roughly the size of France and extends into seven US states and a Canadian province . The fourth - largest river in the United States by volume , the Columbia has the greatest flow of any North American river entering the Pacific . </P>
what river forms part of the border between two northwestern states and flows into pacific ocean
Columbia River