The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
There are so many rises, and so close to you, that it is kind of scary!
While JohnW may be right I'd think more than not what you heard where trout gorging on Green Drake duns. I have witnessed numerous occassions on the upper main where literally every fish, not just trout, are rising in the black of night to the emerging Ep. guttalata nymphs and duns. There are so many rises, and so close to you, that it is kind of scary!
I put the flash light on the river many times only to see some small spinners and a few caddis.