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).
