Mississippi
Spain forms the eastern boundary of Portugal.
The Indian Ocean forms the eastern boundary of Africa.
The southern half of the eastern boundary of Alabama is the Chattahoochee River.
Spain is the only country to border Portugal to the east.
the country that forms the northern boundary of the United States is Canada :)
Minnesota.
The Atlanic Ocean
The states of Iowa, Missouri, and Arkansas have their eastern borders on the Mississippi River. The Mississippi River borders seven other states.
The Connecticut River
The Mississippi River is the East boundary for Arkansas.
The country is Eritrea, and it's eastern boundary is formed by the Red Sea.