Mesopotamia [Gk: ‘(the land) between the rivers’] is defined by the Euphrates (west) and the Tigris (east) and their tributaries. Today, this area is mostly divided between Syria and Iraq, with Iraq having the major share. The two rivers rise in east Turkey: the Euphrates (2700 km long, 1200 km in Iraq) then follows a southwesterly course until it turns southeastwards to flow through Syria; the Tigris (1700 km long, 1400 km in Iraq) almost immediately flows southwards. When they cross the Iraqi northwest border the two rivers are some 400 km apart. From here they flow towards each other until near Baghdad they approach to within 32 km of each other. But here they separate until merging at Qurna to form the Shatt-el-Arab, which discharges into the Persian Gulf.
The flooding each spring of the Euphrates and Tigris and their tributaries has deposited the layers of alluvial sediments that form the Lower Mesopotamian Plain, a fertile region that begins at Ramadi northwest of Baghdad and stretches over 700 km to the Gulf. The floods came too late for winter crops and too early for summer crops so the water had to be controlled by a system of canals, reservoirs and dykes. To create and maintain this irrigation system required cooperation between communities, a condition that naturally led to both local strife and political unity.
To the southwest the western desert served as a natural barrier. The Gulf and large areas of marsh along the Tigris River form the southeastern border of the alluvial plain. To the northeast rises the Jebel Hamrin, a range of hills that indicates the beginning of the Zagros Mountains. To the northwest the flat alluvial plains give way gradually to the Upper Mesopotamian Plain (Jazira).
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