Because of its unique history, many of the boundaries of the political divisions of the United States were artificially constructed (rather than permitted to evolve and drawn using natural features of the landscape). Therefore, many U.S. states have straight lines as boundaries, especially in the West. However, there are many partial state boundaries, particularly in the Midwest, Northeast, and South, that are defined by rivers; in fact, only five states (Colorado, Hawaii, Montana, Utah, and Wyoming) completely lack any borders defined by rivers or waterways.
