log

root. wet (and soft), soaked, swampy

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).