A noun in The Etymologies of the 1930s glossed “torrent, mountain-stream” derived from primitive ᴹ✶ulda (Ety/ULU), where the o became u via a-affection and the ld became ll as usual.
Noldorin
thórod
noun. torrent
thórod
noun. torrent
oll
noun. torrent, mountain-stream
old
noun. torrent, mountain-stream
oll
noun. torrent, mountain-stream
celeg
adjective. swift, agile, hasty
lhagr
adjective. swift, rapid
The form lhegin in the published Etymologies might be a misreading for lhegrin, see VT/45:25. As noted by Bertrand Bellet, the two forms are listed side by side, and they may simply be doublets, but it is also possible that we have here a singular followed by its plural.
A noun for “torrent” in The Etymologies of the 1930s based on the root ᴹ√THOR “come swooping down” (Ety/THOR; EtyAC/THOR). In The Etymologies as published in The Lost Road Christopher Tolkien gave the form as thórod (LR/393), but Carl Hostetter and Patrick Wynne corrected this to thôrod in their Addenda and Corrigenda to the Etymologies (VT46/19). However, in normal Sindarin/Noldorin orthography ô is only used for monosyllables, so thórod is the expected form.