Ah Midge, that's it. The Mayflies are larger, not what we were seeing this last weekend.
And of course I had my "pimp lights" on the bridge turned on for some of that, so could well be I was partly at fault anyway. OTOH, there were also boatloads of the things even before I did the light show thing.
Seems to me the often show up in high humidity and low (no) breeze. Hence my thought about blowing them off from time to time with a shop vac or similar. I tried some battery-operated portable fans, but I think the result wasn't concentrated enough...
Chesapeake Bay, USA