“The route to there goes through a steppe. The area between the Euphrates and Tigris is so fat and abundant that it is said that cattle are driven away from there not to die from over-satiety.
The reason for such fertility is irrigation: the moisture from the rivers through water-bearing veins is spread throughout the soil. These rivers themselves flow down from the mountains of Armenia and, on the way, diverge from each other significantly.
The measured maximum distance between the rivers near the Armenian mountains was 2,500 stadia (392,5 km or 243,6 mi). These rivers, entering the lands of the Medians and the Phrygians, begin to converge, and the further they flow, the narrower the strip of land between them gets.
The closest of all they converge on the plain which is called “Mesopotamia” by the locals, enclosing it from two sides.”
Quintus Curtius Rufus, “The story of Alexander of Macedon”