For those of you who follow such things, we have a hatch today on Mill Creek, Walla Walla Co., WA. Elevation 1100', water temperatue about 53 degrees. Air was upper 70's.
Leptophlebia, western black quill, I believe. This mayfly had a 10mm body.
The adult showed up early afternoon. This evening it is becoming a spinner. Edit, it flew off at 7:00pm. Pretty sure ID correct, the stream by the house is in the baffled flood control area, so slower, cobble bottom with lots of vegetation. Crawler Mayfly. I've been seeing more the last few years, thats why I knew I could follow it's progress. Good to know our stream rehabilitation efforts are working.
Did some follow up research. Nymphs will grow in size late summer and fall and overwinter nearly mature. The hatch has been fairly regular for a few years so I'm thinking around here a small then medium size dark hare's ear fall and winter. I believe most area waters will be closed when the hatch occurs most years. Maybe in a really warm winter. The Walla Walla is open but has a different nature so I'll probably find fewer there. I have also seen the occasional back quill at Bennington Lake near the creek, my guess a stray, but something to look for as it is open sometimes by now.
We have hatches of Leptophlebia in North Idaho Lakes. On CdA Lake they peak around mid-May. When I worked in CdA, I would head out during my lunch hour, find cutties rising to Black Quills and catch a few.