From what I know, it is not a river that forms much of the border. It is the Bitterroot mountain range.
The 42nd parallel forms the southern boundary of Idaho. It marks the border between Idaho and Nevada as well as a portion of the border between Idaho and Oregon.
The Columbia River forms a border between the states of Washington and Oregon.
michigan
The Columbia River
Columbia river
Columbia river
The Columbia River lies between the States of Washington and Oregon, and forms the border between the two.
The Columbia River.
The Columbia River.
The Columbia River.
The Columbia River forms most of the border between Oregon and Washington before flowing into the Pacific Ocean.The river that forms part of the border between two northwestern states and flows into the pacific ocean is the Columbia river.
British Columbia.