Years ago I was camping on a small island in the Mississippi when this happened. There were so many they literally darkened the sky. Needless to say we caught no fish that weekend.
The boat launch in Savanna was being cleared of the dead ones with a snow plow.
L
Living in Dubuque during high school. I witnessed the mayfly phenomena. Remember bridge over Mississippi being closed because they made it slick as ice. They only live a short time and the millions of stacked up rotting corpses put up a beyond wretched stench.