The Zend Avesta, Part I (SBE04): Vendîdâd: Fargard I

The first land, the Airyana Vaêgô by the Vanguhi Dâitya, remained to the last a mythical region. It was originally the abode of Yima and of the righteous, that is to say, a particular form of paradise (see Introd. IV, 38, and Farg. II). Later on, it was looked for in the countries north of Adarbaijan, probably in order that it should be as near as possible to the seat of the Zoroastrian religion, yet without losing its supernatural character by the counter-evidence of facts. This brought about the division of the Vanguhi Dâitya into two rivers: as the Airyana Vaêgô was localised in the country north of Adarbaijan, the river in it must become identified with the Araxes (Aras); but, at the same time, it continued to surround the world eastward under the name of Veh (Vanguhi), which was the Sassanian name of the Oxus--Indus 1. It seems that in the time of Herodotus, the Araxes and the Oxus were considered one and the same river 2, as the Oxus and the Indus were later on; this would account for his strange statement that the Araxes, which is confessedly with him the Oxus or Yaxartes, springs from the land of the Matianians, like the Gyndes, and flows eastwards (I, 202; IV, 40; cf. III, 36; IV, 11); and, at the same time, this would account both for how the Airyana Vaêgô could be localised in the basin of the Araxes and how the Oxus could flow eastwards to fall into the Arabian sea 3.