Boundaries designate the geographic extent of a country are known as borders. Borders mark where one country ends and another begins. When a geographic features such as a mountain, lake, or a river is used to mark the boundaries, it is known as a natural border.
According to one study published in 2020, rivers account for 23% of international boundaries. Rivers also form 17% of state and provincial borders worldwide, and 12% of all county-level local borders.
Map of Major Rivers that Form International Borders
NASA created a map from the study that highlighted the international borders formed from major rivers. All of the blue lines represent major rivers that serve as international borders.
Examples of Rivers as Borders
Here are some examples of where a river serves as the border between two countries.
The Guadiana River separates Portugal from Spain
This photo taken from the International Space station shows the Guadiana River. The Guadiana River empties into the Atlantic Ocean’s Gulf of Cádiz and serves as the border between Portugal’s (right) and Spain’s southern coastal regions (left).
The Guadiana travels east to west through Spain and south through Portugal, eventually forming the Spanish-Portuguese border. The river empties into the Gulf of Cádiz, a section of the Atlantic Ocean, between Vila Real de Santo António (Portugal) and Ayamonte (Spain).
Paraná River and the Argentina–Paraguay border
The 4,880 kilometer-long (3,030 miles) river flows in a northeast to southwest direction across Brazil, Paraguay, and Argentina.
A section of the Paraná River forms the border between Argentina and Paraguay as seen in the map below.
Detroit River Forms a Border Between the United States and Canada
The Detroit River separates the cities of Detroit and Ontario as serves as part of the international border between Canada and the United States.
Belle Island in the middle of the river is part of the United States.
Meado, A. (2018, September 2). River of the strait. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/92692/river-of-the-strait
Popelka, S. J., & Smith, L. C. (2020). Rivers as political borders: a new subnational geospatial dataset. Water Policy. https://doi.org/10.2166/wp.2020.041
Voiland, A. (2020, September 17). When rivers are borders. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/147238/when-rivers-are-borders