The falls from S. Nen Hithoel past Tol Brandir (LotR/366, 373). It was translated as “Roaring Spray” in Tolkien’s “Unfinished Index” of The Lord of the Rings (RC/327). Its initial element is some derivative of the root ᴹ√RAW “*roar” and the second element is ross¹ “spray”.

Possible Etymology: The precise language of this name is unclear, but most of the nearby geographic features are named in Sindarin, as were the earlier forms of this name, making it likely this is a Sindarin name as well. In his Outline of Phonology from the 1950s, Tolkien gave this name as Rauros(se) “roaring-rain”, related to Q. raurō “lion” (PE19/99). This hints that this name might be Quenya, but I believe this example merely indicates the Quenya/Sindarin equivalents of this name: Q. Raurosse and S. Rauros.

Conceptual Development: In Lord of the Rings drafts from the 1940s, the first name for these falls was N. Rosfein >> Rhosfein (TI/273, TI/283), revised to N. Dant-ruin >> Dant-ruinel (TI/283, TI/285) and finally to N. Rauros (TI/285, 316), the last of these names translated “Rush-rain, Roar-rain”.

Earlier name for S. Rauros in Lord of the Rings drafts from the 1940s (TI/285), also appearing as Dant-ruin (TI/283). Its first element seems to be dant “*fall”, as suggested by Roman Rausch (EE/2.39), but the meaning of the second element is unclear.

Earliest name for S. Rauros in Lord of the Rings drafts from the 1940s (TI/283), also appearing as Rosfein (TI/273). Its seems to be a combination of rhoss¹ “rain” and fein “white”, as suggested by Roman Rausch (EE/2.39).