From Proto-Balto-Slavic *rīˀtás, likely continuing Proto-Indo-European *h₃riH- (“flowth, raise”) + *-tós. From the same root, but via different extensions, descend Proto-Slavic *rěka (“river”), Latin rīvus (“river”), Proto-Germanic *rīsaną (“to rise”). The meaning probably went from “to flow, to pass” to “morning” through the idea of the time when the day starts to flow, to pass (or maybe the idea of the sun rising up at daybreak). Cognates include Latvian rîts, Old Church Slavonic ристати (ristati, “to move hastly back-and-forth”).