The Karun (, , the Ancient Greek Eulaeus or Εὐλαῖος, Hebrew Ulai ) is the Iranian river with the highest water flow, and the country's only navigable river. It is long. The Karun rises in the Zard Kuh mountains of the Bakhtiari district in the Zagros Range, receiving many tributaries, such as the Dez and the Kuhrang. It passes through the city of Ahvaz, the capital of the Khuzestan Province of Iran, before emptying to its mouth into Arvand Rud (Shatt al-Arab).
The Karun continues toward the Persian Gulf, forking into two primary branches on its delta – the Bahmanshir and the Haffar – that join the Arvand Rud, emptying into the Persian Gulf. The important Island of Abadan is located between these two branches of the Karun.
Basin
thumb|Catchment area of Karun River (in deep green)
thumb|Karun River near the city of [[Ahvaz]]
The largest river by discharge in Iran, the Karun River's watershed covers in parts of two Iranian provinces. The river is around long and has an average discharge of . The largest city on the river is Ahvaz, with over 1.3 million inhabitants. Other important cities include Shushtar, Khorramshahr (a port), Masjed-Soleyman, and Izeh.
Much of Khuzestan's transport and resources are connected in one way or another to the Karun. Since the British first discovered oil at Masjed Soleyman, the Karun has been an important route for the transport of petroleum to the Persian Gulf, and remains an important commercial waterway.
Water from the Karun provides irrigation to over of the surrounding plain and a further are planned to receive water.
History
thumb|Terracotta vessel with geometrical decorations. Susa I period (4200–3800 BC) - [[Louvre Museum]]
Karun River lies between the Susa plain, the location of the ancient cultures dating back to the fifth millennium BC and, to the south, the newly discovered ancient culture of the Zohreh River plain from the same period. The principal ancient site being excavated near the Zohreh River is Tal-e Choga Sofla <small>(fa)</small>, which has many parallels with Susa.
Later, the Karun valley was also inhabited by the Elamite civilization which rose about 2,700 BC. At several points in history, Mesopotamian civilizations such as Ur and Babylon overthrew the Elamites and gained control of the Karun and its surroundings in modern Khuzestan. However, the Elamite empire lasted until about 640 BC, when the Assyrians overran it. The city of Susa, near the modern city of Shush between the Dez and Karkheh rivers, was one of their largest before it was destroyed by the invaders.
thumb|right|220px|Karun at night
The first known major bridge across the river was built by the Roman captives that included its emperor Valerianus in the Sassanid era, whence the name of the bridge and dam Band-e Kaisar, "Caesar's dam", at Shushtar (3rd century AD).
In two of several competing theories about the origins and location of the Garden of Eden, the Karun is presumed to be the Gihon River described in the Biblical book of Genesis. The strongest of these theories, propounded by archaeologist Juris Zarins, places the Garden of Eden at the northern tip of the Persian Gulf, fed by the four rivers Tigris, the Euphrates, Gihon (Karun) and Pishon (Wadi al-Batin).
In 1888, during a period of increasing British influence in southern Iran, Lynch Brothers opened the first regular steamship service on the river linking Khorramshahr and Ahvaz.
The name of the river is derived from the mountain peak, Kuhrang, which serves as its source.
The film documentary, Grass: A Nation's Battle for Life (1925), tells the story of the Bakhtiari tribe crossing this river.
thumb|left|Derelict vessels and a bridge over the Karun in [[Khorramshahr]]<!--despite the name of the image, this is the Karun-->
It was here during the Iran–Iraq War that the Islamic Republic of Iran Armed Forces stopped the early Iraqi Armed Forces advance. With its limited military stocks, Iran unveiled its "human wave" assaults which used thousands of Basij (Popular Mobilization Army or People's Army) volunteers.
In September 2009, three districts of Basra province in southern Iraq were declared disaster-hit areas as a result of Iran's construction of new dams on the Karun. The new dams resulted in high levels of salinity in the Arvand Roud (Shatt al-Arab), which destroyed farm areas and threatened livestock in that Iraqi Basra area. Civilians in the area were forced to evacuate.
Dams
thumb|right|220px|The [[Karun-3 dam, one of the many large hydroelectric power dams on the Karun River]]
There are a number of dams on the Karun River, mainly built to generate hydroelectric power and provide flood control. Gotvand Dam, Masjed Soleyman Dam, Karun-1 (Shahid Abbaspour Dam), Karun-3, and Karun-4, most of them owned by the Iran Water and Power Resources Development Co., are all on the main stem. Karun-2 would potentially be located in the Sussan Plain between Shahid Abbaspour and Karun-3, but the project is still under consideration because of concern over submerging archaeological sites.
A Karun-5 dam upstream of Karun-4 has also been proposed. The Masjed Soleyman, Shahid Abbaspour, and Karun-3 dams each generate 1,000–2,000 MW of power to service the peaking power sector of Iran's electricity grid, and when completed, Karun-4 will also generate 1,000 MW. There are also many dams on the river's tributaries. Dez Dam, Bakhtiari Dam (under construction) and Khersan-3 Dam (under construction) are among them. Khersan 1, Khersan 2, Zalaki, Liro, Roudbar Lorestan, Bazoft, and others are proposed.