The Paraná River
The southernmost border was the one between southern Egypt ad Numidia
The Uruguay River forms the western border of Uraguay with Argentina.
The Uruguay River forms the western border of Uraguay with Argentina.
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The Paraná and the Iguazu Rivers form the natural border between Argentina and Paraguay.
the "rio de la plata" (a river)
The river that forms the southernmost boundary of Texas is the Rio Grande. It flows along the border between Texas and Mexico, stretching approximately 1,885 miles from its source in the San Juan Mountains of Colorado to the Gulf of Mexico. The Rio Grande serves as a natural border, playing a significant role in the geography and politics of the region.
The Republic of Paraguay is bordered by Argentina to its southern border. Towards is northwest is Bolivia, and to the east and northeast is Brazil.
The physical feature that forms Uruguay's western boundary with Argentina is the Uruguay River. It also forms part of the Brazilian-Argentine border.
Rio Grande forms some of the border.
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The Mississippi River forms Iowa's eastern border, the Missouri forms the western border with Nebraska while the Big Sioux river forms the border with South Dakota.