Well, they're all brownish mayflies with 3 tails, non mottled wings, and a large hind wing.
A size 16 is small for a hendrickson, large for a blue quill. It's a smallish freestone stream, being on the small side of normal seems more likely, and me personally, without measuring, am more likely to underestimate size (call a 14 a 16, vs. an 18 a 16). Timing fits reasonably well for both, they overlap. All suggestions make sense here, but I'm still leaning hendrickson.