The longest river that Bulgaria has a bank on is the Danube (2,888 km), which spans most of the country's northern border. The longest one to run through the country (and also the deepest) is the Maritsa (480 km), while the longest river that runs solely in Bulgaria is the Iskar (368 km).
Most of the rivers that rise in southern Bulgaria have their mouths in the Aegean Sea outside Bulgarian territory. A notable exception is the Iskar that takes its source from Rila and runs through Stara Planina forming a gorge to reach the Danube. Depending on their location, they are divided into two regions, a Western Aegean and Eastern Aegean one. The former embraces the Struma and Mesta, while rivers in the latter include the Arda, Maritsa and Tundzha.