Maritsa (märēˈtsä) [key], river, c.300 mi (480 km) long, rising in the Rila Mts., W Bulgaria, and flowing SE between the Balkans and Rhodope Mts., past Plovdiv, to Edirne, Turkey, where it turns south to enter the Aegean Sea near Enez. The Tundzha is its chief tributary. The Maritsa's lower course forms part of the Bulgarian-Greek border and the Greek-Turkish line. The upper Maritsa valley is a principal east-west route in Bulgaria. The unnavigable river is used for power production and irrigation. It is known as the Évros by the Greeks and the Meriç by the Turks.