The Snake River
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.
That would be Oregon
The Rio Grande
Yes, Idaho does have a panhandle. It forms the northwest portion of the state, and borders Canada on the north and Washington (and Oregon) on the west.
The state that separates Oregon and Washington is Idaho. Idaho is located to the east of Oregon and Washington, serving as a buffer between the two states. The Snake River forms part of the border between Idaho and Oregon, while the state of Washington is located directly north of Idaho.
Columbia River, Oregon.
michigan
Columbia river
Columbia river
The Snake river flows east for part of its course and the water eventually flows into the Pacific Ocean. It forms part of the border between Idaho and Oregon The Columbia river forms most of the border between Washington and Oregon, but the water in it flows WEST, so unless you meant something like "runs in an east-west direction" the Columbia is not the river you are looking for.
The Columbia River.
The Columbia River.