The Strymonian or Strymonic Gulf (Greek: Στρυμονικός Κόλπος, romanized: Strymonikos Kolpos), also known as the Orfano Gulf (Greek: Κόλπος Ορφανού, romanized: Kolpos Orfanou), is a branch of the Thracian Sea—itself part of the Aegean Sea—lying east of the Chalcidice peninsula and south of the Serres regional unit. It was formerly known as the Gulf of Rendina, in reference to the ancient colony of Eion along the gulf's shores.
The river Struma/Strymon empties into the gulf.
The Thracian Sea is the northernmost part of the Aegean Sea. It is bounded by Macedonia and Thrace as well as northwestern Turkey. It connects to the Black Sea through the Dardanelles Straight, the Marmara Sea, and the Bosphorus Straight.
The entire area of the sea lies north of the 40th parallel. Its length from east to west is from 23°E to about 25.8°E, or from the Strymonian Gulf east to the northernmost part of the Gallipoli peninsula and the width from north to south is about 40.25°N to 41°N, or from the Dardanelles north to the boundary between the Xanthi and the Rhodope regional units.
The Struma or Strymonas (Bulgarian: Струма, romanized: Struma, pronounced [ˈstrumɐ]; Greek: Στρυμόνας, romanized: Strymónas, pronounced [striˈmonas]) is a river in Bulgaria and Greece. Its ancient name was Strymon (Greek: Στρυμών, romanized: Strymṓn, pronounced [stryˈmɔːn]). Its drainage area is 17,330 km (6,690 sq mi), of which 8,670 km (3,350 sq mi) in Bulgaria, 6,295 km (2,431 sq mi) in Greece and the remaining 2,365 km (913 sq mi) in North Macedonia and Serbia. It takes its source from the Vitosha Mountain in Bulgaria, runs first westward, then southward, forming a number of gorges, enters Greece near the village of Promachonas in eastern Macedonia. In Greece it is the main waterway feeding and exiting from Lake Kerkini, a significant centre for migratory wildfowl. Also in Greece, the river entirely flows in the Serres regional unit into the Strymonian Gulf in Aegean Sea, near Amphipolis. The river's length is 415 kilometres (258 miles) (of which 290 kilometres (180 mi) in Bulgaria, making it the country's fifth-longest and one of the longest rivers that run solely in the interior of the Balkans.
Parts of the river valley belong to a Bulgarian coal-producing area, more significant in the past than nowadays; the southern part of the Bulgarian section is an important wine region. The Greek portion is a valley which is dominant in agriculture, being Greece's fourth-biggest valley. The tributaries include the Konska River, the Dragovishtitsa, the Rilska River, the Blagoevgradska Bistritsa, the Sandanska Bistritsa, the Strumitsa, the Pirinska Bistritsa and the Angitis.