The Karkheh or Karkhen (perhaps the river 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 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 Karkheh or Karkhen (perhaps the river 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 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.