The Balkan mountain range chain divides Bulgaria into the watersheds of the Black Sea and the Aegean Sea.

An excerpt from the article 34 facts about Bulgaria

In the Black Sea basin, the longest river is the Danube (length within Bulgaria - 471 km), which is a border river with Romania, along with its tributaries: Lom, Ogosta, and Iskyr. Iskyr is Bulgaria's longest river flowing entirely within its borders. The largest rivers in the Aegean Sea basin are Maritsa, Mesta, Kamchiya, and Struma (Strymónas).