chapter also provides information on water use in iran which does not share the watercourse of the tigris river itself but hosts important tributaries within the tigris basin finally the shatt al arab river is discussed together with two additional major tributaries the karkheh and the karun rivers which discharge directly into the mesopotamian marshes or the shatt al arab itself and are hence neither part of the euphrates or tigris river basins chap the keban dam is located km downstream in a narrow gorge