The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
Wow! great photos.
It looks like a sulfur to me.
Few studies have addressed macroinvertebrates in streams below major dams in the Sierra Nevada foothills. Burdick (1974) studied the Kings River macroinvertebrate fauna below the Pine Flat Reservoir (fig.4B). He sampled two sites at 2-week intervals from September 1973 to October 1974. At least 85 species were collected. The caddisfly genus Hydropsyche dominated the biomass. Based on collections of free-flying adults, Burdick (1974) stated that six species of Hydropsychidae were present below the dam, but subsequent work established that only Hydropsyche californica was present. The adults of the other five caddisfly species invaded the Kings River from nearby perennial or intermittent streams (Gill, 1982).