The Rio Grande river forms part of the border between which two countries?

The Rio Grande forms a long section of the international border between the United States (the state of Texas) and Mexico (the states of Chihuahua, Coahuila, Nuevo Leon and Tamaulipas).

Key facts about the Rio Grande:

• It is about 3,050 km long, one of the longest rivers in North America.
• In Mexico it is called the Rio Bravo (or Rio Bravo del Norte), meaning "fierce river".
• It rises in the San Juan Mountains of southern Colorado, USA.
• It flows south through New Mexico, passing Albuquerque.
• From El Paso (Texas) and Ciudad Juarez (Mexico), it forms the border for about 2,000 km, flowing south-east.
• It empties into the Gulf of Mexico near Brownsville (Texas) and Matamoros (Mexico).
• Its largest tributary is the Pecos river in the USA; the Conchos river joins it from Mexico.

Importance:

It supplies water for irrigation, cities and industries on both sides of the border, in a mostly dry region.

Water use is shared under treaties between the two countries, such as the 1944 Water Treaty. Disputes sometimes arise during droughts.

Large dams include the Elephant Butte Dam in New Mexico and the Amistad and Falcon dams on the border.

Political importance:

• The Rio Grande became the border after the Mexican-American War (1846-48), under the Treaty of Guadalupe Hidalgo.
• It is often in the news because of migration across the USA-Mexico border.

The rest of the border:

West of El Paso, the USA-Mexico border runs over land through deserts to the Pacific Ocean near San Diego and Tijuana.

Why not the other options?

The USA and Canada share the world's longest international border, which includes the Great Lakes and the St. Lawrence river.

Mexico and Guatemala share a border partly formed by the Usumacinta and Suchiate rivers.

Colombia and Panama are separated by the Darien Gap, a region of jungle and swamps.