Nestos

Nestos, or Mesta, is a river in the Balkans that crosses two countries. Particularly, it starts from Bulgaria, then goes to Greece and at the end it drains into the Aegean Sea. Its length is 230 km.
Line.
It starts between the southwestern area of the mountainy place called Rila and of the northwestern Rhodope Mountains in Bulgaria. Then it goes through the Pirin mountains and it gets a lot of water from other smaller rivers. It crosses the valley of Nevrokopi and continues between Orvilos and Rhodope, entering Greece through Drama Prefecture. After that it goes to Xanthi Prefecture and cosses a narow and long valley. Eventually at the end, it goes through Xanthi and Chrysoupoli and it drains in the Aegean Sea, making a big delta. 
Usefulness.
It is a natural border between Macedonia and Thrace. It is an important river for the people there because there is a valley so they can water their crops or drink water, take a bath, do watersports etc. Greece and Bulgaria have been arguing for many years about Nestos. After a signed agreement in 1995 the copyrights of Bulgaria are 71% and Greece's 29%.