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 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.) References Geography of the United States Borders of U.S. states