Sources

The term ruighe is nearly always masculine. Except for two outliers, (Allt na Ruighe in Sutherland and Allt ba /ruighe Lèithe in Ross) the only place where ruighe appears as feminine (using OS maps for evidence), is a small area in Glen Urquhart centering around the name Abhainn na Ruighe Duibhe and related names.

This is supported by Dwelly, who gives masculine for this form. Other sources such as Watson’s place-names of Ross and Cromarty (e.g. Allt an ruigh’ shleaghaich) and Robertson MSs also only have ruighe as masculine in place-names.