The country is Eritrea, and it's eastern boundary is formed by the Red Sea.
Sudan is the most eastern country to Eritrea.
Spain forms the eastern boundary of Portugal.
The southern half of the eastern boundary of Alabama is the Chattahoochee River.
The Atlanic Ocean
Minnesota.
The Connecticut River
Mississippi
Mississippi
The Mississippi River is the East boundary for Arkansas.
Two rivers partially form Illinois State's Eastern boundary. The Wabash River is one river that forms Illinois' boundary. The Ohio River is the other river that forms the state's boundary.
Mississippi River