the Rhine river
The Mississippi River forms the western border of Illinois.
The Mississippi river forms the western border of the southern colonies.
because it just did
The Mississippi River forms the Western border of the States of Illinois, Tennessee, and Mississippi. The Mississippi River forms the Eastern border of the States of Iowa, Missouri, Arkansas, and Louisiana.
The Great Smoky Mountains form the eastern border with North Carolina.
The Missouri River and its tributary, the Big Sioux, form the western border of Iowa.
The two mountain ranges generally considered to form the distinction between Europe and Asia are the Urals and the Caucasus. With rivers it's trickier, since there have at times been various interpretations of the distinction between Europe and Asia. I use the 'neutral' word 'distinction' since 'border' overstates the preciseness of the changeover.
Wabash River
the chattahoochee
The Ural Mountains and the Ural River in Russia are the mountains and river that form most of the northern boundary between Europe and Asia.
The Ural Mountains that run through Russia and the Ural River, which flows from Russia, through Kazakhstan to the Caspian Sea.
The Wabash River forms the southern portion of Indiana's western border. The northern portion of Indiana's western border is simply a straight line marked on the map. The southern border of Indiana is also a river - the Ohio.