List of 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 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

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 boundary between New Hampshire and Vermont is the west bank of the Connecticut River. This was established as the eastern boundary of New York by a grant of King Charles II in 1664. It was disregarded by Governor Benning Wentworth of New Hampshire, who treated the New Hampshire Grants west of the river as a de facto part of New Hampshire during the years 1649–1764, but King George III put an end to that in 1764. In August 1781, the Continental Congress decided it would recognize the then largely unrecognized state of Vermont, which had been organized in defiance of New York, on condition that Vermont would agree to certain boundaries. In 1782, the legislature of Vermont agreed, but nonetheless Vermont was not admitted to the Union until 1791. In 1933, citing the 1782 legislation, the United States Supreme Court denied the petition from the state of Vermont to make the boundary 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 north of 39° 30' north latitude is the east bank of the Delaware River.^{[citation needed]}
• 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 boundary between Alabama and Georgia, south of West Point, Georgia, is the west bank of the Chattahoochee River at the mean water mark. This was established in an 1860 Supreme Court ruling, Alabama v. Georgia.

List of river borders

• Arthur Kill: New Jersey, New York (tidal strait)
• Big Sandy River: Kentucky, West Virginia
• Big Sioux River: South Dakota, Iowa
• Blackwater River: Virginia, North Carolina
• Bois de Sioux River: South Dakota, Minnesota, North Dakota
• Brule River: Michigan, Wisconsin
• Byram River: Connecticut, New York
• Catawba River: North Carolina, South Carolina
• Chattahoochee River: Alabama, Florida, Georgia
• Chattooga River: Georgia, South Carolina
• Colorado River: Arizona, Nevada, California, Baja California
• Columbia River: Washington, Oregon
• Connecticut River: New Hampshire, Vermont
• Delaware River: New York, Pennsylvania, New Jersey, Delaware
• Des Moines River: Iowa, Missouri
• Detroit River: Michigan, Ontario
• Great Miami River (mouth only): Ohio, Indiana
• Halls Stream: New Hampshire, Quebec
• Hudson River (lower part only): New Jersey, New York
• Kill Van Kull: New Jersey, New York (tidal strait)
• Menominee River: Michigan, Wisconsin
• Mississippi River: Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana
• Missouri River: South Dakota, Nebraska, Iowa, Missouri, Kansas
• Montreal River: Michigan (Upper Peninsula), Wisconsin
• Niagara River: New York, Ontario
• Ohio River: Illinois, Indiana, Ohio, Kentucky, West Virginia
• Palmer River: Rhode Island, Massachusetts
• Pawcatuck River: Connecticut, Rhode Island
• Pearl River: Mississippi, Louisiana
• Perdido River: Florida, Alabama
• Pigeon River: Minnesota, Ontario
• Pine River: Minnesota, Ontario
• Piscataqua River: Maine, New Hampshire
• Pocomoke River: Maryland, Virginia
• Poteau River: Arkansas, Oklahoma
• Potomac River: Maryland, Virginia, D.C., West Virginia
• Poultney River: Vermont, New York
• Rainy River: Minnesota, Ontario
• Red River of the North: North Dakota, Minnesota
• Red River of the South: Texas, Oklahoma, Arkansas
• Rio Grande: New Mexico, Texas, Chihuahua, Coahuila, Nuevo León, Tamaulipas
• Runnins River: Rhode Island, Massachusetts
• Sabine River: Texas, Louisiana
• St. Clair River: Michigan, Ontario
• St. Croix River (Maine-New Brunswick): Maine, New Brunswick
• St. Croix River (Wisconsin-Minnesota): Minnesota, Wisconsin
• St. Francis River (Missouri-Arkansas): Arkansas, Missouri
• St. Francis River (Quebec-Maine): Maine, Quebec
• St. John River: Maine, New Brunswick
• St. Lawrence River: New York, Ontario
• St. Louis River: Minnesota, Wisconsin
• St. Marys River (Florida-Georgia): Florida, Georgia
• St. Marys River (Michigan-Ontario): Michigan, Ontario
• Salmon Falls River: New Hampshire, Maine
• Savannah River: South Carolina, Georgia
• Snake River: Idaho, Washington, Oregon
• Tennessee River: Kentucky, Tennessee, Mississippi, Alabama
• Tug Fork River: Kentucky, West Virginia, Virginia
• Tugaloo River: Georgia, South Carolina
• Wabash River: Illinois, Indiana
• Montreal River: Michigan, Wisconsin

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.)

List of U.S. states with river borders

Nearly every U.S. state border has some portion that is a river or other water way; 44 in total.

• Alabama
• Arizona
• Arkansas
• California
• Connecticut
• D.C.
• Delaware
• Florida
• Georgia
• Idaho
• Illinois
• Indiana
• Iowa
• Kansas
• Kentucky
• Louisiana
• Maine
• Maryland
• Massachusetts
• Michigan
• Minnesota
• Mississippi
• Missouri
• Nebraska
• Nevada
• New Hampshire
• New Jersey
• New Mexico
• New York
• North Carolina
• North Dakota
• Ohio
• Oklahoma
• Oregon
• Pennsylvania
• Rhode Island
• South Carolina
• South Dakota
• Tennessee
• Texas
• Vermont
• Virginia
• Washington
• West Virginia
• Wisconsin