“The Tigris–Euphrates river system is a large river system in Western Asia that flows into the Persian Gulf. Its primary rivers are the Tigris and Euphrates, along with smaller tributaries…..the two rivers descend through valleys and gorges to the uplands of Syria and northern Iraq and then to the alluvial plain of central Iraq. Other tributaries join the Tigris from sources in the Zagros Mountains to the east. The rivers flow in a south-easterly direction through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf. The rivers and their tributaries drain an area of 879,790 square kilometres (339,690 sq mi) including almost the entire area of Iraq as well as portions of Turkey, Syria, Iran and Kuwait” (Wiki) Nb. The Karun River, the largest river by discharge in Iran, flows from the Zagros Mountains and joins the Shatt al-Arab separately, downstream of the confluence of the Tigris and Euphrates and is therefore NOT regarded as part of the combined basin. It has been given its own Basin” connection. see map