Karun
The Kārun (also spelled as Karoun) is Iran's most effluent, and the only navigable, river. It is 450 miles (720 km) long. It rises in the Zard Kuh mountains of the Bakhtiari district in the Zagros Range, receiving many tributaries, such as the Dez and the Kuhrang, before passing through the capital of the Khuzestan Province of Iran, the city of Ahwaz.
The Karun continues toward the Persian Gulf, forking into two primary branches on its delta: the Bahmanshir and the Haffar that joins the Shatt al-Arab (Arvand Rud in Persian), 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.
In the Biblical tradition, Karun is to be identified with Pishon, one of the four rivers of Eden/Paradise. The others being Tigris, Euphrates and the Karkheh ("Gihon" of the Biblical story). In the early classical times, Karun was known as the Pasitigris, which may be pointing etymologically to the source of the Biblical name, Pishon. The modern medieval and modern name, Karun, is a corruption of the name, Kuhrang which is still maintained by one of the two primary tributaries of the Karun.