Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
During my previous trip to Slough Creek with a friend in late June 2019, we caught many nice fish on streamers, but the water was still high from snowmelt. I wanted to give it a try during a better time of year for dry fly action, so I returned at the end of July, 2020. I started hiking around 1:30 pm and reached camp at 4:00. A lightning storm that had been skirting around me stopped skirting and struck, so I crawled in the tent for a 45-minute nap. I finally got out to fish at 6:30 pm under cloudy but thunderless skies.
I quickly lost one good fish on a hopper. I also tried for way too long to catch one big fish sipping tiny mayfly duns, none of which came close enough to me to catch and identify. All I got from the fish were one or two splashy refusals. I caught a different, more sporadic riser, about 16" long, on an olive female Trico dun pattern, imitating a tiny BWO. The next pool upstream had more rising fish, but beavers and ducks put them down. I eventually caught a plump 17-incher toward dark on a big ant pattern.
The water was low, clear, and very easy to cross in many places. Lots of sandhill cranes added sound effects that felt appropriate to the place, and two whitetail does and a spike buck wandered within twenty yards of me.