The Rio Grande River
The Colorado River forms part of the southwest boundary of the United States.
the country that forms the northern boundary of the United States is Canada :)
The Rio Grande River
The Colorado River
The Colorado river
The Rio Grande River (Rio Bravo as known in Mexico) forms a natural boundary between the southwestern United States and Mexico; however it is not a waterway as the river is not navigable.
The two rivers that partially form the eastern boundary of the United States are the Mississippi River and the St. Lawrence River. The Mississippi River flows southward from Minnesota to the Gulf of Mexico, serving as a significant boundary for several states. The St. Lawrence River, situated in the northeastern part of the country, forms part of the boundary between the United States and Canada.
The Mississippi River spans the United States from Lake Itasca, Minnesota finally opening into the Gulf of Mexico. The nine pairs of American States for which the Mississippi forms the state boundary are Minnesota-Wisconsin, Iowa-Illinois, Missouri-Illinois,
Mississippi
Indiana and Illinois
The Ohio River forms Ohio's southern boundary. It serves as a natural border between Ohio and West Virginia, as well as Kentucky. This river is a significant waterway in the United States, flowing westward before joining the Mississippi River.
The Rio Grande is shared by the United States and Mexico.