The Nestos or Mesta , formerly the Mesta Karasu (Ottoman Turkish), is a river in Bulgaria and Greece. It rises in the Rila Mountains and flows into the Aegean Sea near the island of Thasos. It plunges down towering canyons toward the Aegean Sea through mostly metamorphic formations. At the end, the main stream spreads over the coastal plain of Chrysoupolis and expands as a deltaic system with freshwater lakes and ponds forming the Nestos delta.The length of the river is 230 km, of which 126 km in Bulgaria and the rest in Greece. It forms some gorges in Rila and Pirin.