The Sulak (, ,
[Lepiev A.S., Lepiev İ.A., Türkçe-Çeçençe sözlük, Turkoyŋ-noxçiyŋ doşam, Ankara, 2003]) drains most of the mountainous interior of
Dagestan northeast into the
Caspian Sea
The Caspian Sea is the world's largest inland body of water, described as the List of lakes by area, world's largest lake and usually referred to as a full-fledged sea. An endorheic basin, it lies between Europe and Asia: east of the Caucasus, ...
. 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 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.
The Sulak proper starts at the junction of the Andi and Avar Koysus. It flows northeast into the broad
Chirkey Reservoir. It then flows northwest through the narrow Miatli canyon and reservoir. It reaches flat country at Miatli and leaves the last mountains just south of
Kizilyurt and enters the Terek-Sulak Lowland about from its source. Here there are meanders, former channels and canals. It flows northeast about , turns east and reaches the Caspian after about . Its delta is a smaller version of the Terek River delta about north. The lower Sulak supplies water to
Makhachkala
Makhachkala, previously known as Petrovskoye (1844–1857) and Port-Petrovsk (1857–1921), or by the local Kumyk language, Kumyk name of Anji, is the capital and List of cities and towns in Russia by population, largest city of Dagestan, Russia. ...
through a canal.

In 1735 the Sulak River was stipulated as defining the boundary between the
Russian and the
Persian empires.
References
{{Authority control
Rivers of Dagestan
Tributaries of the Caspian Sea