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.
I caught this tiny larva without a case, but it seems to key pretty clearly to to Glossosomatidae. From there, the lack of sclerites on the mesonotum points to either Glossosoma or Anagapetus. Although it's difficult to see in a 2D image from the microscope, it's pretty clear in the live 3D view that the pronotum is only excised about 1/3 of its length to accommodate the forecoxa, not 2/3, which points to Glossosoma at Couplet 5 of the Key to Genera of Glossosomatidae Larvae.
This genus contains two subspecies that create localized fishable hatches throughout the West. Timpanoga hecuba hecuba can be important east of the Cascades and Sierra Nevada Mountains. Timpanoga hecuba pacifica can be important on the Coast, as its name implies.
Where & when
In 6 records from GBIF, adults of this genus have been collected during September (50%), August (33%), and July (17%).
In 12 records from GBIF, this genus has been collected at elevations ranging from 4741 to 9360 ft, with an average (median) of 5269 ft.
This specimen is 14 mm. Technically this is the subspecies T. h. hecuba. The Cascades, Sierras and further West is where the other subspecies, T. h. pacifica is found. The Great Basin seems to have formed a barrier preventing any overlap in their distribution.