The Great River of Beleriand (S/120), a combination of sîr “river” and the adjective iaun “wide”, reduced to its suffixal form -ion also seen in the names of lands (PE17/42).
Conceptual Development: This river was named G. Sirion in the earliest Lost Tales (LT1/238) and was explained as an archaic word for “river” in the Gnomish Lexicon (GL/67). The name N. Sirion appeared in The Etymologies from the 1930s as an elaboration of N. sîr (Ety/SIR). The derivation given above appeared in Tolkien’s Words, Phrases and Passages from the Lord of the Rings (WPP) from the late 1950s or early 1960s (PE17/42). In an early name list its Qenya equivalent was given as ᴱQ. Sirion as well (PE13/102).
The “Great River” of northwestern Middle-earth (LotR/52), a compound of and “long” and duin “river”, and thus literally meaning “Long River” (RC/765, VT48/23).
Conceptual Development: This river was first mentioned as N. Anduin in the earliest versions of the “Fall of Númenor” from the 1930s (LR/33). In the earliest drafts of the Lord of the Rings from the 1940s this river was named N. Beleghir “Great River”, but it was quickly changed to Anduin (RS/410). As he composed the Lord of the Rings, Tolkien considered other variants: Sirvinya “New Sirion” (TI/119) and Andon (TI/298), but all were rejected in favor of Anduin.