Mesopotamia, which is Greek for "land between the rivers," was primarily settled in the southern plains of the Tigris and Euphrates rivers. The two rivers serve as Mesopotamia's major sources of water and begin in the eastern portion of Turkey. They run parallel to one another, creating the boundaries of Mesopotamia, and cease as they form the Shatt al Arab river that empties into the Persian Gulf.
In the ancient Mesopotamia the water supply was not regular - be it in the form of rains or any other source which resulted in high temperature and the ground use to become hard and dry and unsuitable for cultivation of plants for almost eight months in a year. Irrigation was central to controlling the crops in southern Mesopotamia. In northern Mesopotamia, agriculture proved successful at an earlier date, dating back to 10th millennium BCE
Irrigation has been an important base for agriculture in Mesopotamia (what is now Iraq and part of Iran) for 6000 years. Mesopotamia has low rainfall, and is supplied with surface water by only two major rivers, the Tigris and the Euphrates.
It was the two rivers that became the basis upon which the wealth of the region was based. Through relatively easy irrigation the agriculture could yield heavy crops. There were fish in the rivers, the area had a diversified agriculture and wildfowl was available out near the coast.
Although they are much smaller than of the Nile, they have much more dramatic spring floods, from snowmelt in the highlands of Anatolia, and they carry more silt. Furthermore, the plains of Mesopotamia are very flat, and poorly drained, so that the region has always had persistent problems with poor soil, drought, catastrophic flooding, silting, and soil salinity.
Farmers didn't know when to plant or how big flood would be as the annual flood was unpredictable, it came sometime between April and June. Drought period without enough rain and snowfall lowered river levels. This made it hard to water crops; people starved if crops failed.
The civilization of Mesopotamia existed for 26 centuries. It was in a position to command by trade or plunder all the resources of the ancient world provided it could keep the vast floodings of the Tigris and Euphrates under strict control. From their earliest writings, the Sumerians recounted the story of their most terrible flood, estimated by historians about 8000 B.C. (The tale perpetuates in the Biblical story of Noah and the Great Flood.)
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Ancient Mesopotamian Water