The name does not appear to be associated with Rearguard Mountain, nor does it appear on early maps. Keddie mentions in the report on his archeological survey, 1971:

In the upper Fraser, Chinook salmon head upstream from July 1 to August 31 and return down-stream from mid April to early June. The migration goes as far as Rearguard Falls (east of Tête Jaune Cache) and spawning occurs in almost every stream flowing into the Fraser below this point.

There is a sign at these falls indicating that they mark the limit of the upward migration of chinook salmon on the Fraser River. Hence the name “Rearguard.”

But according to naturalist Art Carson of Valemount:

The falls are now believed by Canada Fisheries and Oceans Department to be an insurmountable barrier to perhaps 90 percent of the salmon which attempt it, however. Of the successful ones (perhaps 350 to 550 per year) a few have even been found spawning in the glacial waters of the Robson River. As far as is known, Overlander Falls several kilometers upstream from Rearguard marks the true upper limit of migration, with the uppermost spawning beds (only large enough for a few fish) being located just below Overlander Falls adjacent to Denny Hogan’s early railway construction camp.