[1] There were rivers of this name in NW India and in Susis (modern Iran). The latter (cf. mu 570) is the one in question here, since the headword phrase is a reflection of Herodotus' statement (1.188: web address 1) that the Kings of Persia drink their water from no other river.
[2] Not otherwise attested as such. Perhaps the phrase is here simply to clarify that 'Choaspes' is the river's name, not an alternative word for a(ny) river.