Matt,
I'm going to be the contrarian here...It's my nature I guess...
Nowhere in your post did you state that you were unable to see what was the cause of this activity...Your post is shy of observational information...Did you see any bugs what-so-ever?
I think a size 16 PT is almost always a good choice when you may not know what's happening...It's a good place to start anyway.
My guesses are as follows in what I think may be the order of probability with my first choice #1...
Although classified as crawlers, Paraleptophlebia nymphs swim very well. They are generally tolerant of faster water than Leptophlebia and inhabit pockets in riffles as well as moderate runs.
#2...I just got back from Grayling and they are a little behind, hatchwise, for this time of year...I spotted one small black caddis...There are a few different suspects that should be around now or at least soon.
#3...Some of the little early spring stones have been hanging around for a while already and I saw a few different species with a size 18 being the most interesting to me since it's been a while since I've made it up for opening day...They come in a variety of sizes here and I had them crawling all over me...Some crawl towards the shore to emerge and some ride for some time on the current and seem to hardly move until they want to fly off. I have seen small brookies leave the water and intercept them as they ride along...It almost looks like they are playing and they take them on their way back in to the water...You would have a difficult time seeing them since they are basically flat & dark as they ride along...
The caddis mentioned above can cause splashy rises as they rocket to the surface with a hungry trout in hot pursuit...They seem unaware that there is an end to their environment and can also leave the water...
The Paraleps are pretty good swimers and could cause something similar to a trout chasing a caddis up...A size 16 would be in the ballpark for them and even a Henny for an indiscriminate stocky...
My guesses...
I had a nice hatch last Saturday and it was interesting because most of the fish needed to find calmer water since the river was at flood stage. I sat there and watched as the Hendrickson's were flooding off with the Baetis & Mahoganies blended in...We caught fish on top but not all of them. I pointed out to my angling friend that some of the surface activity was caused as the fish caught a dun on the way up and turned back down to its feeding station...This caused a surface bulge...
My $2.50 cents worth...I know you were out early, but I'm not so sure this matters...When you got to go, you go!The weather over here has been weird enough to hamper what might be considered normal, though the Hennies were like clockwork...I looked like a genius when I kept joking, "Hennies at 2:00...Better be in your spot by 2:00." It was like I had them trained. :)
Spence