Tolkien used a similar set of words for “forest” starting with the earliest versions of Elvish, but their derivation evolved somewhat over time. The earliest related root was ᴱ√TAVA “beam” with variant ᴱ√TAFA (the latter marked by Tolkien with a “?” and with no obvious derivatives) from the Qenya Lexicon of the 1910s as the basis for words like ᴱQ. taule “great tree”, ᴱQ. tauno “forest” and ᴱQ. tavar “dale-sprite” (QL/90). It also had derivatives in the contemporaneous Gnomish Lexicon such as G. tavros/G. taur “forest” and G. tavor “wood fay” (GL/69).
ᴱQ. taure “forest” did not appear as an independent word until drafts of the Oilima Markirya from around 1930 (PE16/62; MC/213). Thereafter Tolkien mostly stuck with Q. taurë and N./S. taur for “forest”. In The Etymologies of the 1930s Tolkien gave the root ᴹ√TAWAR “wood, forest” (Ety/TÁWAR), though in one place it was ᴹ√TAR (EtyAC/TUR). In notes associated with the Quendi and Eldar essay from 1959-60 Tolkien gave √TAWA “wood”, and in notes on “large & small” roots from 1968 Tolkien had √TAW “wood” (PE17/115).
Tolkien considered a bewildering variety of roots as the basis for the suffixal element S. -ló “flood” in Sindarin, common in river names such as S. Gwathló and S. Ringló. In a collection of notes associated with the name S. Lhûn from around 1967, Tolkien first considered √SLOUN, √SLON or √SLUN (unglossed); then √(S)LOW “flow freely (fully)” (PE17/136-7; VT48/27-28). In notes on The Rivers and Beacon-hills of Gondor from 1967-69, Tolkien instead wrote:
> 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 except in the intensive formation oloiya- “to inundate, flood”; oloire “a great flood” (VT42/10).
I prefer this last explanation, as it explains a wider variety of words. As for S. Lhûn, in notes from 1968 Tolkien explained it as a loan word from Khuzdul (VT48/24).