Final answer:
The Intermontane Basins and Plateaus region is located between the Rocky Mountains and the Mississippi River, which has been historically misunderstood due to an incorrect notion about the river's northern source.
Step-by-step explanation:
The regional board that lies between the Rocky Mountains and the Mississippi River is known as the Intermontane Basins and Plateaus. This area includes parts of western Colorado, western Wyoming, western Montana, Idaho, Nevada, Utah, and portions of Washington, Oregon, and California in the United States, as well as the southern part of the Canadian Rockies. The misconception that the source of the Mississippi River lay further north had once made the boundary between the Mississippi River and the 49th parallel uncertain.