This root first appeared as unglossed ᴹ√MEN in The Etymologies of the 1930s, where it seemed to be stationary in nature given its derivatives ᴹQ. men “place, spot” and ᴹQ. ména “region” (Ety/MEN). It was an element in direction words like ᴹQ. formen/N. forven, apparently meaning “✱north place” at this conceptual stage. In Demonstrative, Relative, and Correlative Stems (DRC) from the late 1940s, Tolkien first glossed ᴹ√MEN as “place, spot”, but that instance of the root was revised to ᴹ√NOM (PE23/112 and note #12), and a bit later in the same document Tolkien gave a new meaning to ᴹ√MEN as “purpose, aim” (PE23/112).
In the Quenya Verbal System (QVS) from 1948, Tolkien first gave ᴹ√MEN the gloss “aim at, intend, purpose” with the more specific sense “make for, proceed towards” when used with the allative, but then crossed this out and simply made the gloss “go, proceed” (PE22/103 and note #21). In Common Eldarin: Noun Structure of the early 1950s, Tolkien first gave √MEN the gloss as “place, situation, site”, then revised it to “direction, object, point moved toward” (PE21/79 and note #39).
Thereafter Tolkien mostly gave this root the basic sense “go” (PE17/93, 143, 165; VT39/11; VT42/32). Tolkien gave a somewhat lengthy description of the root in Definitive Linguistic Notes (DLN) from 1959:
> √MEN go, move, proceed in any direction (irrespective of speaker’s position, or assumed point of thought). mēn- a way, a going, a mov[ement] (PE17/165).
Thus in the 1950s and 1960s, Q. formen likely meant “✱north way” rather than “✱north place”. However, in a footnote in The Road Goes Ever On from 1967, Tolkien did say that men had the meaning “direction, region” as part of an explanation of √MENEL “heavens” = men + el (“region of stars”), so it seems Tolkien did not entirely abandon the use of this element as a name for places.
Neo-Eldarin: For purposes of Neo-Quenya, I sometimes see √MEN use for movement in general (as in motion picture) rather than movement in a direction. I personally recommend using ᴱQ. lev- “(intr.) move” from Qenya Wordlists of the 1920s for that purpose (PE16/132) since I think it is important to distinguish “move” from “go”, but if you prefer to use only later words you might be more comfortable expanding the semantic scope of √MEN to movement in general.
@@@ 1950s mendë “will” = “✱intent” and 1964 menta- “have as object, (in)tend”
√KEL was established as the root for “flow” very early in Tolkien’s Elvish languages, also with the variant vocalic extension √KELU. ᴱ√KELE appeared in the Qenya Lexicon of the 1910s glossed “flow, run” and also “ooze, trickle”, though the latter sense did not appear in later writing (QL/46). It also had a variant ᴱ√KELU elsewhere compared to ᴱ√QEL+U of similar meaning (QL/75). It had derivatives in both Qenya and Gnomish such as ᴱQ. kelu- “flow”, G. celu- “trickle” and ᴱQ. kelu(me)/G. celu “stream” (QL/75; GL/25).
The root ᴹ√KEL reappeared in The Etymologies of the 1930s where it seems to have an added connotation of “flow downwards”, given its gloss “go, run (especially of water), flow away downhill” (Ety/KEL). This more specific meaning to the root continued to appear in later writings, with glosses such as 1930s “flow, flow away, run (of rivers)” (PE18/58), 1940s “run away especially downwards or at end” (PE22/114), 1950s “go away, flow away or flow down (of water)” (PE18/103), and 1960s “flow (down)” (PE17/157). The u-extension ✶kelu with the (continuative?) sense “flow, well up” remained common throughout all these periods as well (PE18/86; PE22/98, 133, 135).
Starting with The Etymologies of the 1930s, Tolkien compared this root with etymological variants √KWEL “fade (away), die away, grow faint” and √KYEL “come to an end, cease, run out” (Ety/KWEL, KYEL; PE18/58, 103), and indeed Tolkien used it as one of his basic examples of such etymological variations:
> There existed in Quenderin 1. √KEL “flow, flow away, run (of rivers)”. Of this simple base, since the initial variation is possible, while the sundóma is the same 2. √KJEL “cease, come to an end”, and 3. √KWEL “fade, die away, grow faint” may be regarded as differentiated variants (second version of Tengwesta Qenderinwa, TQ2, circa 1950, PE18/103).
A similar note appeared in the first version of Tengwesta Qenderinwa (TQ1) from the 1930s (PE18/58). Likewise ✶kelu was one of his main examples of variant vocalic extensions:
> This “variant extension” always had the form i or u. It appears in many cases where its original function is no longer discoverable, if indeed that was more than to serve as a euphonic connecting link to affixes. But the added element [u] often appears as a differentiator as in kel¹-u beside kel², and some old verbs have a fixed u as the end of their base (TQ2, PE18/86).
In the Quendi and Eldar essay of 1959-60, it seems that √KYEL “end” was replaced by √TEL “close, end, come to an end” (WJ/411) as indicated by the replacement of 1930s ᴹQ. tyelma “ending” (LR/72) by later Q. telma “conclusion” (WJ/411). In a 1967 note the etymological variant √KYEL seems to have been assigned a new meaning “go down slowly”, which served as the basis for Q. tyellë “grade, a step in a stairway or ladder” (PE17/157).
To summarize, the base root √KEL meant “flow (down)” for pretty much all of Tolkien’s life, usually used with a variant vocalic extension √KELU, and starting in the 1930s it had etymological variants √KWEL and √KYEL, with √KWEL always meaning “fade (away), die” and √KYEL meaning “cease, end” from the 1930s to 40s, but changing to “go down slowly” sometime between the 1950 and 1967 (but probably before the 1st edition of LotR), with “end” being reassigned to √TEL by 1960. See the entry on √TEL for further discussion of those developments.