It's hard to say exactly, but a few things come to mind. I assume you can sight a few trout in the water. The submerged vegetation must be home to lots of sow bugs. In the summer, there are probably some terrestrials (ants, crickets, hoppers, inch worms) and maybe damselflies. Try a fly like the Trout Crack or Little Guy underwater and a size 16-22 winged ant on the surface.