The Sulak (Russian: Сула́к, Kumyk: Сулак (Sulak)/Къой-сув (Qoysuw), Chechen: ГӀойсу, romanized: Ġoysu) drains most of the mountainous interior of Dagestan northeast into the Caspian Sea. It and most of its branches flow in canyons. Its main tributaries are, from north to southeast:
- The Andi Koysu flows north-northeast and joins the Avar Koysu to become the Sulak. A few of its upper tributaries are in Georgia. Near the junction of the two rivers were the Siege of Akhoulgo and the Battle of Gimry.
- The Avar Koysu flows northeast to join the Andi Koysu, forming the Sulak. Its upper tributaries are the northeast flowing Khzanor and the northwest flowing Dzhurmut.
- The Kara Koysu flows northeast and joins the Avar Koysu about 10 km (6.2 mi) northwest of Gergebil. North of Gergebil is the Irgan dam and reservoir. Its long upper tributaries include the Karalazuger, Tleyserukh and Risor.
- The Kazikumukh Koysu flows north and joins the Kara Koysu at Gergebil.
North of the Sulak basin is the Terek basin and south of it is the Samur basin. To the west is the crest of the Main Caucasian Range of the Caucasus Mountains and to the east are many short rivers that flow down into the Caspian.
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