This large Mayfly (2" in length including tail) crash landed on our picnic table at a Columbia River campground. We believe it's Green Drake but verification from one of our amateur entomologists would be helpful.
also in vote of a hex dun. i used to see a ton of these near the columbia as they tend to prefer slower (if not still) water. green drake duns tend to have shorter fatter bodies, bluish green darker gray wings and three tails instead of two. the spinners have the characteristic white body that also give it the nickname "coffin fly"...great picture!