A river on the other side of the Misty Mountains from the river Hoarwell (S. Mitheithel). It appeared in The Etymologies as N. Rhimdath < Rhibdath (Ety/RIP), and also appeared as Rhimdath on a very early map of the wilderlands (RS/205). In a later draft map it was labeled Rhimdad (TI/296), but was unlabeled in the maps published in The Lord of the Rings.

According to The Etymologies, its initial element is derived from N. rhib- “to flow like a (?torrent)”, apparently assimilated to Rhim-. Its final element is most likely N. dad “down”. Roman Rausch suggested (EE/1.5) that its earlier form Rhimdath may instead have N. dath as its second element, perhaps with the meaning “steep fall, abyss” (EtyAC/DAT).

If this name was adapted to the phonology of later Sindarin, it might be S. *Rimdad.