Néstos River, river in southwestern Bulgaria and western Thrace, Greece. The Néstos rises on Kolarov peak of the Rila Mountains of the northwestern Rhodope (Rodopi) Mountains. The river’s upper confluents separate the Rila and Pirin ranges from the main Rhodope massif. Crossing the Bulgarian frontier into Greece, the Néstos divides Greek Macedonia from Greek Thrace. From just west of Stavroúpolis to its mouth on the Aegean Sea, 150 miles (240 km) from its source, it forms the boundary between Kaválla and Xánthi nomoí (departments). Above Paranéstion, however, the river is confined to inaccessible gorges as it traverses the sparsely populated, mountainous Dráma nomós (department). West of Xánthi the Néstos reaches the marshy, alluvial coastal plain of Khrisoúpolis (Chrysopolis).