In late July, Weather Service radar at Lacrosse, Wisconsin showed a storm breaking. Colors lit up the screen, but there was no rain anywhere close.
The radar was picking up not a storm of raindrops, but an explosion of mayflies hatching on the Mississippi River dividing Minnesota and Wisconsin. This hatch featured the species Hexagenia bilineata, which can thrive only in a clean stream.
Hatches shrank in the 1960s and 1970s due to pollution. The Clean Water Act and other laws have drastically reduced the sewage and toxic pollutants that threatened the species.
Now that they’re back, riverside communities know what that means: after mating on their hours or few days of life outside of burrows in the river bottom, they die off in terrific numbers, sometimes enough to require shoveling. A 1960 study for Iowa State University reported that “Drifts of the insects form under street lights at such times, traffic is impeded, shoppers desert the streets, and, in extreme cases, snow plows are called out to reopen highway bridges which have become impassable.”
But mayflies are a keystone of the river’s ecology and a prime food for fish.
The bug swarm may not be as charismatic as bald eagles fishing in the Mississippi, but it means the same thing – environmental laws are working, and in many ways our rivers and streams are getting cleaner and healthier.
A one-day dieoff is a big nuisance, but a dying river is a profound tragedy. Today, the mayfly shows that the Mississippi River is recovering.