The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
As usual, I reserve the right to be completely wrong.:)
There were enough of these flitting around in the evening to keep the fish looking up. Anglers can easily confuse these with Dicosmoecus species (October Caddis) at this time of year, but their wings are quite different and they like gentler flows.