River borders of U.S. states

Because of its unique history, many of the boundaries of the political divisions of the United States were artificially constructed (rather than being 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 a number of states, particularly in the Midwest, North and South with at least partial river borders.

Typically the boundary is the "thread of the channel", under a rule that the United States inherited from England, where it applies to boundaries between counties. There are at least four exceptions, where the boundary is one bank of the river rather than the thread of the channel:

The course of the Charles River was used to indirectly define the border between Massachusetts and Rhode Island. 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.)

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