A noun for “rivulet” in The Etymologies of the 1930s derived from the root ᴹ√SIR “flow” (Ety/SIR).
Qenya
kelut
noun. rivulet
siril
noun. rivulet
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A noun for “rivulet” in the Declension of Nouns of the early 1930s (PE21/33, 35), clearly a derivative of ᴹ√KEL(U) “flow”.