Bahmanshir in the context of Karun River

The Karun (Persian: کارون, IPA: [kɒːˈɾuːn]), the Ancient Greek Eulaeus (Greek: Εὔλαιος or Εὐλαῖος, Hebrew Ulai (Hebrew: אולי), is the Iranian river with the highest water flow, and the country's only navigable river. It is 950 km (590 mi) 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.

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. The port city of Khorramshahr is divided from the Island of Abadan by the Haffar branch.

Haffar is a canal in Iran.

During the early Islamic centuries, the Daylamite Buwayhid king, Panah Khusraw Adud ad-Dawlah, ordered the digging of a canal to join the Karun River, which at the time emptied independently into the Persian Gulf through the Bahmanshir channel, to the Shatt al-Arab waterway (known as Arvand Rud in Iran), the joint estuary of the Tigris and Euphrates rivers. The extra water from the Karun, which, at times during the spring melt, discharged over 27 times the volume of the Tigris-Euphrates water that reaches the Shatt al-Arab) makes the joint estuary more reliably navigable.

Abadan (Persian: آبادان; pronounced [ʔɒːbɒːˈdɒːn]) is a city in the Central District of Abadan County, Khuzestan province, Iran, serving as capital of both the county and the district. The city is in the southwest of the county. It lies on Abadan Island (68 km or 42 mi long, 3–19 km or 2–12 miles wide). The island is bounded in the west by the Arvand waterway and to the east by the Bahmanshir outlet of the Karun River (the Arvand Rood), 53 kilometres (33 mi) from the Persian Gulf, near the Iran–Iraq border. Abadan is 140 km from the provincial capital city of Ahvaz.