Final answer:
Bulgaria has its northern border at the Danube River, making it a significant natural boundary in Eastern Europe.
Step-by-step explanation:
The Danube River serves as a northern border to certain countries in Eastern Europe. It is Bulgaria which has its northern border at the Danube River. This river is significant not only for its role in defining political boundaries but also as a major waterway for transport and trade. The Danube flows across Europe, influencing the landscapes and economies of the nations it traverses, including Romania, where it creates a natural border with Bulgaria and Serbia before completing its journey into the Black Sea.