Tigris–Euphrates river system through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf. The region has historical importance as part of the Fertile Crescent region, in which civilization is believed to have first emerged. The ecoregion is characterized by two large rivers, the Tigris and Euphrates. The rivers have several small tributaries which feed into the system from shallow freshwater lakes, swamps, and marshes, all surrounded by desert. The hydrology of these vast marshes is extremely important to the ecology of the entire upper Persian Gulf. Historically, the area is known as Mesopotamia. As
Euphrates enters the Upper Mesopotamian plains, its grade drops significantly; within Syria the river falls while over the last stretch between Hīt and the Shatt al-Arab the river drops only . The Euphrates receives most of its water in the form of rainfall and melting snow, resulting in peak volumes during the months April through May. Discharge in these two months accounts for 36 percent of the total annual discharge of the Euphrates, or even 60–70 percent according to one source, while low runoff occurs in summer and autumn. The average natural annual flow of the Euphrates has been determined from