Looks like Callibaetis with those speckled wings.
https://www.google.com/search?q=cal...bih=923&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi-2bSlhY_MAhUHwmMKHdruDcAQ_AUIBigB

 

Yeah but no Stillwater nearby.

 

View attachment 115407 View attachment 115408
West Seattle creek 4/13/16
Big group of spinners (imagos) at the bridge over the creek
Rhithrogena?

Rhithrogena looks right

http://www.west-fly-fishing.com/entomology/mayfly/march-brown.shtml

 

Oh shoot I think maybe they were duns or subimagos then.

 

West Seattle creek 4/13/16
Big group of spinners (imagos) at the bridge over the creek
Rhithrogena?

Hi Matt B-

Yes, very well done. I believe both photos to be male imagoes (spinners) of genus Rhithogena.

 

Just for reference, here are pictures of a Callibaetis dun (subimago) and a spinner (imago). Typical of all mayflies, the wings of the dun are translucent, as opposed to the clear (hyaline) wings of the spinner. The spinner also has much longer tails than the dun.

 

Just for reference, here are pictures of a Callibaetis dun (subimago) and a spinner (imago). Typical of all mayflies, the wings of the dun are translucent, as opposed to the clear (hyaline) wings of the spinner. The spinner also has much longer tails than the dun

Ah yes thanks. The mottled wings on the bug I shot (same bug both photos) had me second guessing life stage. But they are clear and mottled I suppose.

 

But do note that the wings are hyaline and glossy (as opposed to translucent and dun-colored). The mayfly subimago came to be called "dun'' because of its dull (as opposed to glossy) wing and body colors; the color dun is defined as "a nearly neutral slightly brownish dark gray and ranging from red to yellow in hue". The most common mayfly in England is Ephemerella danica, whose subimago is a good example of this coloration, and the term came to be applied, by anglers, to all mayflies in the subimago stage of their lives (also the largest hatches of E. danica occur in May in England, which is how mayflies came to be called mayflies).