A noun that served as the final element in various river names. Tolkien was uncertain whether the primitive form of this word began with simple l- or s-prefixed sl-, hence the variation between lô and lhô. In notes on Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s, Tolkien said the non-suffixal form of this word was hlô “flood” (PE17/96). In notes from 1966-67 he gave a bewildering variety of derivations for this word, but mostly represented it as lhô or hlô in various attempts to connect it to the river name S. Lhûn (PE17/136-137; VT48/27-28).

In notes on The Rivers and Beacon-hills of Gondor from the late 1960s, while discussing the river name S. Gwathló Tolkien said:

The element -ló was also of Common Eldarin origin, derived from a base (s)log: in Common Eldarin sloga had been a word used for streams of a kind that were variable and liable to overflow their banks at seasons and cause floods when swollen by rains or melting snow; especially such as the Glanduin (described above) that had their sources in mountains and fell at first swiftly, but were halted in the lower lands and flats. *sloga became in Sindarin lhô; but was not in later times much used except in river or marsh names. The Quenya form would have been hloä (VT42/9).

Tolkien rejected this etymology, however, replacing it with the following:

Lô was derived from Common Eldarin base LOG “wet (and soft), soaked, swampy, etc.” The form *loga produced S. lô and T. loga; and also, from *logna, S. loen, T. logna “soaking wet, swamped”. But the stem in Quenya, owing to sound-changes which caused its derivatives to clash with other words, was little represented ... the Quenya form of S. lô would have been **loa, identical with Q. loa < *lawa “year”; the form of S. loen, T. logna would have been **lóna identical with [Q.] lóna “pool, mere” (VT42/10).

This final etymology appears to be the last one Tolkien wrote on the topic; later in the same document he used the form lô in discussions of the river name S. Ringló (VT42/13-14). As for the meaning of the word, it seems it applied both to wide rivers with a tendency to flood their banks, as well as fenlands or wetlands in general.

Conceptual Development: As an element in river names, -lo first appeared in the river names N. Gwathlo and N. Ringlo in Lord of the Rings drafts (TI/304; WR/287). G. lô “pool, lake” from the Gnomish Lexicon of the 1910s is possible precursor (GL/54).

Neo-Eldarin: For purposes of Neo-Eldarin, I prefer the pre-1968 form of the word lhô, as this is both more distinctive and also allows us to salvage Q. hloä of similar meaning.