I was up in the pasture pool and had the same thing happen. Lots of bows working big mayflies right after the downpour and in with them a couple browns. Their rise was noticably different so I was able to target them. I landed one that was 12 inches, LDR one that was about 18 and rolled one that was at least 20 inches about 8 feet from the canoe. I watched him fin the surface for at least 10 minutes only a rod length away before I got a fly on that he would take but after one miss he was gone. Got some nice 2nd and 3rd year bows that were full of fight.
FFO The fish were taking emergers. Not sure of the name of the fly I was using so I will try to describe it, maybe someone else might know, #14 hook, brownish dubbing, brown hackle palmered, on top of it were about 5 strands of deer hair with a couple strands pointing out the back for a tail and tied off in the front. After this fish chewed it up I switched over to a sort of modified Devil Bug and did just as well, it has a dubbed body tied sparse,not too bushy in the back and trimmed short in the front. These two flies are very close in their outline. I have fished this modified Devil bug in the past with great luck over fish taking emergers it's one of my favorite when all else fails flys chris r