Rivers That Define U.S. International Borders

United States – Canada

Numerous rivers form segments of the world's longest international border:

• Saint Lawrence River
• Niagara River
• Detroit River
• St. Marys River
• Rainy River
• Pigeon River
• Saint Croix River
• Columbia River (portion)

United States – Mexico

The Rio Grande (Río Bravo del Norte) forms the entire 1,255-mile (2,020 km) southern border of Texas with Mexico, from El Paso to the Gulf of Mexico. It is the only river defining the U.S.–Mexico border.

These waterways have historically facilitated trade, transportation, and settlement, while also serving as political boundaries negotiated through treaties.