This specimen appears to be of the same species as this one collected in the same spot two months earlier. The identification of both is tentative. This one suffered some physical damage before being photographed, too, so the colors aren't totally natural. I was mostly photographing it to test out some new camera setting idea, which worked really well for a couple of closeups.
Sporadic hatches are rarely as outstanding as those of Isonychia. On streams with good populations, they are reliably hatching in light numbers, here and there, for most of the evening through most of the mid- to late season.
The spinners, and occasionally the duns, produce more concentrated action, but the real value of the Isonychia hatch is its duration and the size of the flies; large trout become ever watchful for them, even when they aren't emerging.
All the species of Isonychia are similar in appearance and behavior.