The Mississippi river was the border.
The western border of the 13 colonies was formed by the Appalachian Mountains.
The east was bounded by the Mississippi and the West ( andSouth) by the Missouri River.
The western border of Canaan was marked by the Mediterranean Sea.
The river, Rio Uruguay, borders Uruguay's western border.
The Mississippi river forms the western border of the southern colonies.
The Mississippi river forms the western border of the southern colonies.
Andes Mountains.
Colorado River
The Appalachian Mountains marked the Western border of the newly-founded USA.
utah ************************** Note: Arizona's (natural) western border is mostly defined as the Colorado River. (Utah borders to the north!)
The physical feature that forms Uruguay's western boundary with Argentina is the Uruguay River. It also forms part of the Brazilian-Argentine border.