I'd imagine the mayflies you spotted were Hendricksons. I was working on the lower river yesterday and we got to participate in the first real mayfly hatch of the year. We've had some spaoradic hatching but this was a bonafide hatch and the fish were tuned in. The hatch was hendricksons and they were coming off in great numbers. You'll notice with hendricksons that the male and female are quite different from each other. The females are usually a little bigger than the males (12 or so) and their color is kindly a dull pinkish tan. The males come off in sizes closer to 14 and their color is more of a dark body, with your typical quill gordon flies being a good match. They usually hatch out together on the Hiwassee and for the most part the fish will just eat them all. However, from time to time, the fish will pick on one side of the mayflies heavier than others. Yesterday they were really tuned in to the males over the females.
The hatch should move upriver over the next few weeks. THe heavy part of the hatch always starts downriver and moves up, so keep your eyes out for them when your there. I'm sure the rest of the river will still see lot's of sporadic bugs popping off but now that the mayflies are starting....we know the bug celebration is ready to go!