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 Ar… - Gherardo Gnoli

[The horizon of the Avesta] “is according to Burrow, wholly eastern and therefore certainly earlier than the westward migrations of the Iranian tribes.” ...

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.

“in the denomination of Ariana, which became known to the Greeks after the Macedonian conquest of the eastern territories of the old Persian empire, there was obviously reflected a tradition that located the Aryan region in the central-southern part of eastern Iran, roughly from the Hindukus southwards, and that considered some of the Medes and the Persians in the west and some of the Bactrians and Sogdians in the north as further extensions of those people who were henceforth known by the name of Ariani. And this, to tell the truth, fits nicely into the picture we have been trying to piece so far. Here too, as in the passages of the Avesta we have studied from the Mihr Yast and the Zamyad Yast, the geographical horizon is central-eastern and southeastern; the northern lands are also completely peripheral, and Chorasmia, which is present only in the very peculiar position of which we have spoken in the Mihr Yast, is not included.” ...