Final Answer:
The Maritsa River forms the border between Greece and Turkey.
Step-by-step explanation:
The Maritsa River, also known as the Meriç or Evros in different sections, serves as a natural boundary between Greece and Turkey. It originates in the Rila Mountains in Bulgaria and flows southward, eventually forming part of the border between Greece and Turkey before emptying into the Aegean Sea. The river plays a crucial role in delineating the border between the two countries, and its course has historical and geopolitical significance. The region around the Maritsa River has seen various historical events and cultural influences due to its strategic location as a natural border.
In summary, the Maritsa River acts as the border between Greece and Turkey.