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.