In Persian the word for water is "ab" and likewise this is spread among other related Iranic languages that is ultimately derived from the Indo-European Vedic word "ap".
Ossetians deviated from usage of "ab/ap" in favor of "don", their existing word for river.
After the Alano-Scythians migrated into modern-day North Ossetia, several place names derived from this word still exist such as the river Ир-æф (Ir-ap, meaning Iron/Ossetian water) aka the Urukh river. At some point in history "ap" was abandoned and is now rendered an archaic word.
An explanation for this change can be found among other Caucasian peoples, who all have similar words and concept for water and river. For example, the formal Ossetic word for river is Цæугæдон (lit. flowing-water) and likewise in Mingrelian it is წყარმალუ/tsqarmalu (lit. water-flowing), in Ingush it is Додахий/dodakhij, in Abkhaz аӡы-ас, and in Circassian псы-хъо.