The classical and late classical name for the Karun river--one of the four rivers of Paradise or the Biblical Eden. The Old Testament maintains the four rivers to have been, the Euphrates, the Tigris, the Gihon and the Pishon.
The first two are clearly the primary rivers of Iraq and still carry the ancient names. As in the past many millennia, Gihon and Pishon still flowi through Eden---identified with the great marshlands of southern Iraq and Iran. These are the great rivers of Karkheh and Karun, that rub shoulders with the Tigris and Euphrates in volume and importance. They also share their extended deltas--the marshlands--with each other, forming an extended system.
The classical name, Pasitigris, may preserve in its first element the Biblical name, Pishon. The river Pasitigris is mentioned in Plutarch's Lives as where Eumenes a general of Alexander the Great, met the army of Antigonus when he was ill.