This nymph keys to Baetis assuming the villipore is present (hard to see in my photos or scope), and within that genus it tentatively keys to the flavistriga species complex, of which Baetis flavistriga itself is by far the most common in Washington state, so that's the most likely ID.
This mayfly was collected from Mystery Creek #249 on July 6th, 2020 and added to Troutnut.com on July 12th, 2020.
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