I'm basing the ID of this near-mature nymph on a male spinner with similar size and markings, collected on the same trip. There isn't a key to reliably identify the nymphs of most Rhithrogena species otherwise.
This mayfly was collected from Mystery Creek #249 on July 4th, 2020 and added to Troutnut.com on July 12th, 2020.
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Nymph: The juvenile, underwater stages of mayflies, stoneflies, dragonflies, and damselflies and other aquatic insects whose juvenile stages are covered by hard exoskeletons. The word can also refer to the fishing flies which imitate these creatures, in which case it is used as a blanket term for flies imitating any underwater stage of an invertebrate (except for crayfish and leeches).
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.