The Aegean Sea catchment basin drains the Thracian Plain and most of the higher lands to the south and southwest. Several major rivers flow directly to the Aegean Sea. Most of these streams fall swiftly from the mountains and have cut deep, scenic gorges. The 480 km–long Maritsa (of them 321 km in Bulgaria) and its tributaries drain all of the western Thracian Plain, all of Sredna Gora, the southern slopes of the Balkan Mountains, and the northern slopes of the eastern Rhodopes. After it leaves Bulgaria, the Maritsa forms most of the Greco-Turkish border. Maritsa's major tributaries are the Tundzha, Arda, Topolnitsa, Vacha, etc. The other Bulgarian rivers flowing directly to the Aegean are the Struma and the Mesta.