Female Baetid spinners were abundant subsurface looking for substrates to lay their eggs, and they found my waders. I'll update the ID for this picture after identifying the associated male spinner. I caught several fish on a small brown soft-hackle fished as a crude imitation of these spinners before the fish all started taking Tricorythodes spinners instead.
1. The hind wing has three longitudinal veins (Longitudinal vein: Longitudinal veins are the major long veins running length-wise through an insect's wing, connecting the base to the outer margin, or the major branches from those veins.), but the third is faint, short (about half the length of the wing), and close to the wing margin.
2. Then antenna is brown fading into white at the tip, and the base is ringed with white.
3. The joints of the tarsal segments on the middle and hind leg have fine black markings.
Dun: Mayflies have two adult stages. They first emerge from the water as duns (scientifically known as the subimago stage). They then molt into the spinner (imago) stage, in which they mate and die. Sometimes the word "dun" is confusingly used to refer to a brownish gray color in fly tying materials.
Longitudinal veins: Longitudinal veins are the major long veins running length-wise through an insect's wing, connecting the base to the outer margin, or the major branches from those veins.
Ovipositing: To lay eggs.
Spinner: There are two winged stages of adult mayflies. They emerge from the water as duns, molt on land (usually) into their fully mature stage, spinners. As spinners, they mate, lay eggs, and die.