Final answer:
The Danube River forms a natural border between Bulgaria and Romania to the north, before flowing into the Black Sea.
Step-by-step explanation:
The country that has the Danube River forming its border with Romania to the north is Bulgaria. The Danube is Europe's second-longest river and acts as a natural boundary between these two countries before it continues its flow towards the Black Sea.
Not only does the river serve as a border, but it's also an essential waterway for transport and trade, supporting the rich biodiversity within the Romanian forests.