Quintus Curtius Rufus, “History of Alexander the Great”. “The route there goes through steppes. Situated between the Euphrates and Tigris Rivers, they are so abundant that the livestock is driven out from the pasture not to die from the glut of food, as some say.
The reason for such fertility is irrigation: the moisture from both rivers is distributed through the soil via water-bearing vines. The rivers themselves originate in the mounts of Armenia and on the way spread out widely.
Those who measured the maximum distance between the rivers near the mounts counted 2,500 stadia (392.5 km or 243.6 mi, 1 stadia being equal to 157 meters). Entering the areas of the Medians and Phrygians, the rivers begin to come together. The further they flow, the narrower the land between them becomes. They come the closest at the plateau called by the locals “Mesopotamia” and enclose it from two sides.”
Strabo, “Geography”. “To the north of Taurus begin many spurs, one of them being the so-called Antitaurus. Such a name is bore by a mountain that embraces Sophene in a valley located between the mount and Taurus.
On the other side of the Euphrates, closer to Lesser Armenia to the south, behind Antitaurus spans a large mountain and many spurs. One of those spurs is called Pariadrus, the other is named Moschus, the third bears various names. Those mountains grasp the entirety of Armenia from Iberia to Albania.”
Herodotus, “History”. “15 temporary settlements with defensive fortifications are lying over a territory spanning for 561 parasangs (2,642.3 km (1,641.9 mi), with 1 parasang being equal to 30 stadia, according to the definition of the parasang by Herodotus himself).
Four navigable rivers flow through this country. Each of those rivers must be crossed by vessels. The first river is Tigris, the second and the third are called Zabat. Those are two different rivers originating in different areas. The first Zabat flows from Armenia, while the other from Matiene. The fourth river is called Gind. This river was divided by Cyrus into 360 channels back in the times.
The route carries on to the country of Cissia where 11 temporary settlements are located on a territory spanning for 421 parasangs (1,982.9 km or 1232.1 mi) up until the river Khoaspa, which is also navigable. The city of Sus lies on the river. Between Sardis and Sus, there are 111 temporary settlements and an equal number of inns.”
Avienus, “Description of the World’s Lands”. “Further are the Tibareni, above them are the Chalybes, in whose lands the fields rich in deadly iron are resounded with the bang of the high anvils. Beyond them spread out the Assyrian lands along with Theriodont, which originates in the Armenian ridge and neighbors the fields of the Amazonid tribe…”