an important river in the southwestern United States; rises in the Rocky Mountains of northern Colorado and flows southwest through Utah into Arizona (where it flows through the Grand Canyon) and then southward through the southern tip of Nevada, then forming the border between California and Arizona and finally into Mexico where it empties into the Gulf of California; the main source of water in the southwestern United States | a river that rises in the Rocky Mountains in Colorado and flows southeast through Kansas and Oklahoma and through Arkansas to become a tributary of the Mississippi River |