They traditionally tend to be obstacles that make it difficult for opposing armies to pass, they're also natural landmarks, making it easier for the book keepers and mapmakers.
Spain's borders do no follow rivers.
because they follow rivers
They are formed as borders because they are formed naturally.
They are formed as borders because they are formed naturally.
The rivers
Tigris and Euphrates Rivers.
I Follow Rivers was created on 2011-01-21.
Rivers
There are four rivers that provide natural borders for the state of Florida. They are the Flint River, the Chattahoochee River, Saint Mary's River, and the Perdido River.
because they form a natural barrier
lakes
The Atlantic Ocean and rivers.