The Hilmand region and the Hamun-i Hilmand are beyond all doubt the most minutely described countries in Avestan geography. ... - Gherardo Gnoli

We do not know for certain which river of the traditional cosmology it corresponded to, although the one that naturally comes to mind is the river Arodvi Brzi. Likewise we are unsure of its identification with this or that river of actual geography, although in this case, too, it does not seem that we have a great many rivers to choose from. For a number of reasons that we cannot go into here, as it would lead us too far astray the only rivers we can seriously take into consideration are those such as the Oxus and the Helmand... I have already collected on several occasions evidence and arguments in favour of the latter river in particular...

Given its very Oriental horizon, this list must be pre-Achaemenid; on the other hand, the remarkable extendedness of the territories concerned recommends situating them in a period much later than the Zoroastrian origins. (…) one or several centuries later than Zarathuštra’s preaching.

Now, the obstacle that arises from the theory of the Medes and the Persians having emigrated from the North ... on the basis of a highly conjectural interpretation of the archaeological evidence, is now removed by the archaeologists themselves. A body of evidence... orients us in quite a different direction, namely towards that of a migratory movement, probably a slow, progressive one, from East to West, along the great Khorasan Road.....