The longest river running entirely within Bulgaria and flowing as a Danube tributary is the Iskar.
The Iskar is about 368 km long, beginning in the Rila Mountains and heading north through Sofia Province before reaching the Danube near Gigen. Its course is unusual because it crosses the Balkan Mountains, the major watershed dividing Bulgaria’s Danube and Aegean drainage basins. The river is also regarded as the oldest river in the Balkans geologically, having preserved its general direction through major landscape changes.
The river passes through or near Samokov, Sofia and Mezdra, and fills the Iskar Reservoir, Bulgaria’s largest artificial lake. Farther north, it cuts the scenic Iskar Gorge, an important transport corridor containing roads and railway lines.
A common mix-up is the Maritsa, which is longer overall, and the Danube, which is Bulgaria’s longest river by total length. Neither runs entirely within Bulgaria: both cross or border multiple countries. Yantra is also a Danube tributary, but at roughly 285 km it is shorter than the Iskar.