Rivers as Borders

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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.

Map showing major rivers in the world that form international borders.
Map showing major rivers in the world that form international borders. Source: NASA.

Almost half of the international borders in South America are formed by rivers. In particular international river borders of Brazil, Colombia, Venezuela, Suriname, Guyana, French Guiana, Bolivia, Paraguay, and Uruguay are all notable.

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).

A photo taken from the International Space Station showing the Guadiana River that separates Spain from Portugal.
The Guadiana River as seen from space. Portugal is on the right and Spain is on the left. Photo: International Space Station, NASA, public domain.

Paraná River and the Argentina–Paraguay border

The Paraná River is South America’s second longest river. The The Paraná River is South America’s second longest river. The river flows in a northeast to southwest direction across Brazil, Paraguay, and Argentina.

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.

A map showing rivers in a region of Paraguay, Argentina, Brazil, and Uruguay where rivers form international borders.
The Paraná River forms part of the International border between Argentina and Paraguay. Map: NASA, public domain.

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.

A photograph taken from space showing the Detroit River separating the cities of Detroit and Ontario.
The Detroit River serves as the international border between Canada and the United States in this area. Image: International Space Station, NASA, public domain.

References

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 Policyhttps://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

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