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. 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 completely lack any borders defined by rivers or waterways, as well as Hawaii whose borders are the islands.

The rule of the thread of the channel and its exceptions

River boundaries are typically defined by the "thread of the channel", under a rule that the United States inherited from England, where it applies to boundaries between counties. 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. 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. 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. 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.
The boundary between Delaware and New Jersey north of 39° 30' north latitude is the east bank of the Delaware River.
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 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
Arkansas River: Colorado, Kansas, Oklahoma, and Arkansas
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 : Ohio, Indiana
Halls Stream: New Hampshire, Quebec
Hudson River : New Jersey, New York
Kill Van Kull: New Jersey, New York
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, 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. The Merrimack River defines part of the border between Massachusetts and New Hampshire, which runs parallel to the river, three miles north of it

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.