The Karun is Iran's most effluent and only navigable river. It is 950 km (590 mi) 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 Ahvaz before emptying to its mouth into 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 Arvand Roud, 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. Wikipedia