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The Karkheh or Karkhen (known as the Gihon—one of the four rivers of Eden/Paradise to the Bible and as the Choaspes in ancient times; also called Eulæus; Hebrew: אולי Ulai) is a river in Khūzestān Province Andimeshk city, Iran (ancient Susiana) that rises in the Zagros Mountains, and passes west of Shush (ancient Susa), eventually falling in ancient times into the Tigris just below its confluence with the Euphrates very near to the Iran-Iraq border. In modern times, after approaching within 10 miles of the Dez River, it turns to the southwest and then, northwest of Ahvaz, turns northwest and is absorbed by the Hawizeh Marshes that straddle the Iran/Iraq border. Its peculiarly sweet water was sacred to the use of the Persian kings. Ancient names for the Karkheh should be treated as conjectural because the bed of the river has changed in historic times, and because a nearby watercourse between the Karkheh and the Dez River, the Shaur, confuses the identification. 
The problem with the ancient names is that while the Karheh flows a mile or two west of Susa, another major watercourse flows parallel to the Karkheh within a few miles east of Susa. When these rivers are in flood stage, the entire area south of Susa can be flooded, as the waters of the two watercourses mingle. The watercourse a mile or two east of Susa, now called the Shaur, flows east between the Haft Tepe and Shaur ridges into the Dez River, north of where the Dez and Karun rivers merge. At some previous time, the Karkheh may have joined the eastern end of the Shaur. The timing of these changes is not known with any certainty. The ancient name of the Shaur may have been the Choaspes.
^Potts, Daniel T. (1999). The Archaeology of Elam: Formation and Transformation of an Ancient Iranian State. Cambridge University Press. p. 236. ISBN0-521-56496-4. "One refers to his camp while on campaign and to action near the Ulai, the modern Karkheh river, in Khuzistan"
^John Hansman, "Charax and the Karkheh," Iranica Antiqua VII(1967), pp.21-58, and Michael J. Kirkby, "Land and Water Resources of the Deh Luran and Khuzistan Plains," Appendix I, in Studies in the Archaeological History of the Deh Luran Plain. The Excavation of Chagha Sefid, edited by Frank Hole, pp. 251-288. Memoirs, no. 9, Ann Arbor, Museum of Anthropology, University of Michigan
^"In the third year of the reign of king Belshazzar a vision appeared unto me, even unto me Daniel, after that which appeared unto me at the first. And I saw in the vision; now it was so, that when I saw, I was in Shushan the palace, which is in the province of Elam; and I saw in the vision, and I was by the river Ulai." (Daniel, 8:2)
^"And I heard a man's voice between the banks of Ulai, which called, and said, Gabriel, make this man to understand the vision." (Daniel, 8:16)