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 four mainland states (Colorado, Montana, Utah, and Wyoming) completely lack any borders defined by rivers or waterways, as well as Hawaii whose borders are the islands.
Map of U.S. river/waterway state borders (highlighted)
The rule of the thread of the channel and its exceptions[edit]
River boundaries are typically defined by the "thread of the channel" (the river's thalweg, usually in the approximate middle of the river's channel), under a rule that the United States inherited from England, where it applies to boundaries between counties.[citation needed] In the United States, there are at least six exceptions, however, where the boundary is one bank of the river rather than the thread of the channel:
The boundaries between Kentucky and West Virginia and the three states to their north – Ohio, Indiana, and Illinois – is based on the historical northern bank of the Ohio River.[1] In 1763, Britain defeated France in the Seven Years' War, whose North American theater was called the French and Indian War. At that time, Canada, which had been a French colony, became a British colony, and Parliament made the north bank of the Ohio the southern boundary of Canada. The river was thus included in the district of Kentucky, which was then a part of Virginia.[citation needed] In January 1980, the U.S. Supreme Court ruled in Ohio v. Kentucky that the state line is the low-water mark of the Ohio River's north shore as of Kentucky's admission to the Union in 1792.[2] Because both damming and natural changes have rendered the 1792 shore virtually undetectable in many places, the exact boundary was decided in the 1990s in settlements among the states.[3]
The boundary between Delaware and New Jersey south of a certain point is the east bank of the Delaware River, rather than the thread of the channel.
The boundary between Maryland and Virginia is the south bank of the Potomac River. This also applies both to the border between Maryland and West Virginia (from Harper's Ferry to the source of the Potomac near the Fairfax Stone) since the latter was at one point part of Virginia, and to the border between Virginia and Washington, D.C., since the capital was established from a section of Maryland property.
The course of the Charles River was used to indirectly define the border between Massachusetts and Rhode Island.[citation needed] The Merrimack River defines part of the border between Massachusetts and New Hampshire, which runs parallel to the river, three miles north of it (see Northern boundary of Massachusetts.)
