Might want to do some research here. http://www.troutnut.com/hatch/7/Mayf...ia-Hendrickson
Jason's website is crammed full of great images. My gut is small PT's. Another recommend I came across was a wiggle nymph. (Sounds like something you'd want to put down below something heavy and let it wiggle and bounce as you swing it. There's gotta be a fave mayfly nymph pattern you like that will mimic this - difficulty is that there's several *stages* of colors. Brown to olive from what I'm seeing.
Does sound like a lot of fun, though.