California's southern border is shared with the country of Mexico.
The southernmost border was the one between southern Egypt ad Numidia
Rio Parana
Parana R.
Rio Grande forms the Southern border of the United states.
The Paraná River
The Mississippi river forms the western border of the southern colonies.
The Aquidaban River is a river in northeastern Paraguay. The Apa River forms part of the border between Paraguay and Brazil. The Acaray River in eastern Paraguay.
The gulf that forms part of the southern border of the United States is the Gulf of Mexico
Arkansas
The Paraguay River runs through the city of Asunción, which is the capital of Paraguay. This river forms the border between Argentina and Paraguay.
Paraguay takes its name from the Paraguay River. The river runs through Brazil and forms part of that country's border with Bolivia as well as running through Paraguay and forming part of the border with Argentina. It flows 2,549 km south before meeting the Parana River. The name is taken from the Guarani words para (ocean) gua (to or from) and y (water); ie water that goes to the ocean.