A verb in the Quenya Verbal System (QVS) from 1948 given as the main example of a u-verb, with glosses like “flow”, “begin to flow” and “spring forth (of water)” and derived from the root √KEL “flow” (PE22/97, 110, 114). It seems that originally the u-extension to this verb was simply to make the form more distinct: “In addition many bases of simple √TAL form made verbs of a different sort, employing a fixed vocalic suffix (ū, or ā̆) after the base: as √KEL: kelu ‘flow, well up’ (PE22/98)”. But the u-verb extension often had an inceptive sense (indicating the beginning of things), so that the sense of celu- shifted: “But is also outside tense associated with the notion of ‘begin to do something now (completed later)’ so in u-verbs: kelu, begin to flow (PE22/97)”.

Tolkien discussed this at length in the section of QVS on u-verbs:

The sense-difference between these and normal verbs is not so clear, since an additional -u seems often to have been added as a mere formative: to increase the phonetic content of the stem, as liru- “sing (gaily)”; or to distinguish between bases of identical (or similar) form with different senses, as kelu- “spring forth (of water)”: √KEL, √KYEL, run away especially downwards or to an end, cf. kelya “(it) sends running down = it rains”.

But often as in case kelu- above this suffix marks the beginning (more or less sudden) of an action that may continue later, and was thus anciently and still in Quenya often conjoined with the prefix ete/et- “out”: so in the ancient ektelu- (< etkelu) “gush forth” (PE22/114).

As such, I think this verb primarily has the sense “flow forth or spring forth (of liquids)”, indicating either a sudden new flow or the beginning place of an ongoing flow, such as Anduinë celu i Míse Orontillon “Anduin flows forth from the Grey Mountains”. For a more ordinary continuous “flow”, I think the verbs sirya- or [ᴹQ.] sir- would be used. I also think the related verb [ᴹQ.] ehtelu- < ᴹ✶ektelu- originally had the sense “*flow out of an entrance”, so generally from underground hence = “well or bubble up”; see that entry for discussion.

Conceptual Development: ᴱQ. kelu- “flow” dates back all the Qenya Lexicon of the 1910s where it appeared under the early root ᴱ√KELE or ᴱ√KELU “flow, run; ooze, trickle” (QL/46), but there is no indication that it had an inceptive sense in the 1910s or 20s.