They are formed as borders because they are formed naturally.
To shutcho biscuit
Spain's borders do no follow rivers.
The rivers
Tigris and Euphrates Rivers.
Rivers
Some examples of natural borders separating countries include rivers (such as the Rio Grande between the US and Mexico), mountain ranges (like the Andes between Chile and Argentina), and bodies of water (like the English Channel between the UK and France). Oceans, deserts, and forests can also act as natural borders.
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.
because they follow rivers